PCB protection for 480VAC mains

[P E Schoen]

As mentioned in my other post on Jim Thompson's "Free Consulting" thread, I
am designing a new version of a special SCR trigger board. On this version,
I want to have the option of taking the zero crossing reference for initial
phase angle firing, from the voltage across the SCRs, rather than from a
safer, more current limited, 120 VAC control circuit. The mains supply may
be 480 VAC rated at 400A or more.

Pretty much all small fuses are rated at no more than 250 VRMS, and the
smallest 600V class are 1.5" long and 0.41" diameter:
http://www1.cooperbussmann.com/pdf/da894d76-f42a-42a3-bce0-f90d61f488ee.PDF
They are also about $15 each for 1 amp fuses

I only need about 10mA for the reference, so I am considering fusible
resistors, which are rated at 500V but typically only up to 220V mains. Here
are details:
http://www.welwyn-tt.com/pdf/datasheet/EMC.PDF

The 33 ohm version is rated at 2W nominal or about 250 mA, and the fusing
chart shows a maximum 5 second trip time at 100W, or 1.74A. I am mostly
concerned about safety in case of a major fault, so it would be OK if the PC
board traces would blow off cleanly, or components would be destroyed. But
the danger is if it does not interrupt the fault and arcing causes a chain
reaction of damage and destruction.

This test set is not required to pass UL testing or other certification. The
circuitry is safely enclosed in a 14 gauge steel cabinet, and is operated by
trained technicians who unrack metal clad switchgear for testing and then
rack it back in the cubicles, which is probably much more hazardous than
operating a test set which produces only up to about 25 VRMS (but up to
60,000 amps). Yet safety is always a concern.

The ideas I have are:
(1) External metering PT rated for 480 to 120 VAC for the reference.
(2) External 600V fuses
(3) Several small 250V PCB fuses in series.
(4) Fusible resistors as described above
(5) Rely on thin PCB tracks as fusible links
(6) Don't worry, be happy, (go lucky)
http://en.wikipedia.org/wiki/Don't_Worry,_Be_Happy

Paul
www.pstech-inc.com

 

[P E Schoen]

Lasse wrote in message

something like this:
http://www.newark.com/littelfuse/04...a-slow-blow/dp/34C6799?in_merch=Popular Fuses
?

Wow, that seems perfect. I did a general search and I've looked some time
ago, but either it's fairly new or I just missed it.

And I'm getting an order together from Newark, so I'll add a few of those
babies.

Thanks!

Paul

 

[Joerg]

Lasse wrote in message


Wow, that seems perfect. I did a general search and I've looked some
time ago, but either it's fairly new or I just missed it.

And I'm getting an order together from Newark, so I'll add a few of
those babies.

Thanks!

If they don't pan out you could ask Vishay if they'd endorse their 500V
fusible resistors for 600V:

http://www.vishay.com/doc?30232

These guys have 1000V fusible resistors, scroll to the end:

http://www.tokenonline.net/resistor/fusible-resistor.htm

 

[P E Schoen]

"Joerg" wrote in message

If they don't pan out you could ask Vishay if they'd endorse their
500V fusible resistors for 600V:

These guys have 1000V fusible resistors, scroll to the end:

http://www.tokenonline.net/resistor/fusible-resistor.htm

Welwyn also has some rated at 1000V. But Newark doesn't stock them:
http://www.welwyn-tt.com/pdf/datasheet/SPH-SPF.PDF

Their selector shows them as "level 69V" and the EMC2 as "level 11.7V". I
don't know what that means.
http://www.welwyn-tt.com/products/r...sp?application=Fusible&technology=%&package=%

Actually 500V is probably enough. 600V is more of a device class, but some
fuses like the KTK are 600V, and the same size fuses in slow blow, FNQ, are
rated 480V.

480 VAC service is usually 277 to ground, for wye connections, but it could
be delta, and a phase could be as high as 480 VAC to ground. Our new test
sets are not rated for 600V input, but older ones are. And they typically
have a tap switch that has a maximum setting of 600 VAC, even on 208V input.

Mostly I worry about a fault from line to ground, or something like the 120
VAC control voltage. In that case, depending on phase, a 480V supply could
be 600V to ground in some cases. There is no way to protect against all
possibilities. There are also two gate/cathode pairs that come onto the
board. Maybe I need to fuse those as well.

But my customer does not believe in anything more than the most basic
protection. The earlier test sets had something like ten fuses rated 30-60A
600V, to protect a set of relays, and they have been removed. We also have a
voltage relay which does not allow the test set mains to be energized unless
the power inputs are properly configured for 208/240 or 480 VAC. But we had
some problems with the VRLY that disabled the test set and it had to be
jumpered out until it could be repaired, so the technician could use it. So
now my customer sees any protective device as a potential liability, and
says he would rather have a test set heavily damaged by an incorrect
connection, than have it falsely prevent the technician from using it.

Management vs Engineering politics. Dilbert, anyone?

Paul

 

[Glen Walpert]

As mentioned in my other post on Jim Thompson's "Free Consulting"
thread, I am designing a new version of a special SCR trigger board. On
this version, I want to have the option of taking the zero crossing
reference for initial phase angle firing, from the voltage across the
SCRs, rather than from a safer, more current limited, 120 VAC control
circuit. The mains supply may be 480 VAC rated at 400A or more.

Pretty much all small fuses are rated at no more than 250 VRMS, and the
smallest 600V class are 1.5" long and 0.41" diameter:
http://www1.cooperbussmann.com/pdf/da894d76-f42a-42a3-bce0- f90d61f488ee.PDF
They are also about $15 each for 1 amp fuses

The voltage rating of the fuse is not all you need to consider with high
power circuits, you also need to consider the AIC (Amps Interrupting
Current) rating for fuses or circuit breakers used in high power circuits
where the available fault current exceeds 10 kA, which is usually the
case in industrial power systems.

The current fuse in a Fluke DMM is only rated for 10 kAIC, for instance.
I heard the bang (at least as loud as a shotgun, caused hearing damage to
the two closest people) and saw the remains of the meter when someone
accidentally contacted a 480 volt circuit with about 18 kA available
fault current with the meter set for current measurement. The fuse
failed to clear, and the probes substituted as fuses, with the probe tips
evaporating well back into the plastic probe handle. We opened up the
meter and found most of the traces completely evaporated and most leads
blown out of IC packages.

The guy with the meter got lucky, the probes were far enough apart that
the arcs did not join for a direct line to line arc, which could have
caused a potentially fatal arc blast.

The easy way to check the upper limit on available fault current is to
divide the current rating of the transformer supplying power by it's
impedance factor (both will be on the nameplate). Typical impedance
factors are around .05 (or lower) (sometimes listed as a percentage), so
the available fault current is limited to about 20 times (or more) the
nameplate current. The actual fault current will be lower, usually not
much, due to other factors which quickly become rather tedious to
calculate and are not normally bothered with unless the simple upper
limit is just over a standard AIC rating.

Specifying parts not rated for the available fault current has been known
to cause injury and death, I suggest you not do it.

Regards,
Glen Walpert

 

[P E Schoen]

"Glen Walpert" wrote in message

The voltage rating of the fuse is not all you need to consider with high
power circuits, you also need to consider the AIC (Amps Interrupting
Current) rating for fuses or circuit breakers used in high power circuits
where the available fault current exceeds 10 kA, which is usually the
case in industrial power systems.

The current fuse in a Fluke DMM is only rated for 10 kAIC, for instance.
I heard the bang (at least as loud as a shotgun, caused hearing damage
to the two closest people) and saw the remains of the meter when
someone accidentally contacted a 480 volt circuit with about 18 kA
available fault current with the meter set for current measurement.
The fuse failed to clear, and the probes substituted as fuses, with
the probe tips evaporating well back into the plastic probe handle.
We opened up the meter and found most of the traces completely
evaporated and most leads blown out of IC packages.

The guy with the meter got lucky, the probes were far enough apart
that the arcs did not join for a direct line to line arc, which could have
caused a potentially fatal arc blast.

The easy way to check the upper limit on available fault current is
to divide the current rating of the transformer supplying power by
it's impedance factor (both will be on the nameplate). Typical
impedance factors are around .05 (or lower) (sometimes listed as
a percentage), so the available fault current is limited to about 20
times (or more) the nameplate current. The actual fault current
will be lower, usually not much, due to other factors which quickly
become rather tedious to calculate and are not normally bothered
with unless the simple upper limit is just over a standard AIC rating.

Specifying parts not rated for the available fault current has been
known to cause injury and death, I suggest you not do it.

You raise some excellent points, and I am familiar with the concepts of
available fault currents, interrupting rating, and impedance factors (also
sometimes known as regulation, in %). Since you brought this up, it will be
helpful to discuss the details of these test sets and their safety issues.

First, there is no way of knowing what sort of mains supply it will be
connected to, in the field, so we must assume the worst, which would be
probably a 480 VAC supply with a 400A or even 1200A capacity, and it is very
possible that the source may be of even higher kVA. We once took our largest
test set to an electrical generating station and connected the input as
solidly as possible to their largest service, and we were able to obtain
more than 100kA on the output into a shorting bar, with an open circuit
voltage of about 12 VAC. This was a test set rated about 6000A continuous,
so a rough estimate of the impedance factor (or regulation) was about 5%, as
you said, with the output about 20x nominal. The current draw on the mains
would be about 1/40 that of the secondary, or about 2500 amps.

The early versions of these test sets were protected with only a MCB rated
at 200 to 400 amps, with interrupting rating of about 12,000 amps. Our newer
test sets have a pair of LPS-200 fuses which are rated at 200kAIC, and that
should be sufficient for most normal services which are protected by current
limiting fuses. When an extreme overload occurs, such fuses trip in less
than 1/2 cycle, even before the first peak, so they limit the actual
"let-through" current to something that a MCB can handle, and in fact will
probably stop the current before the breaker even has time to operate. I
have seen a movie comparing the performance of fuses and MCBs with various
available fault currents, and even at the rated IC, the breaker was badly
damaged, but it did its job of clearing the fault. At much higher fault
currents, however, the breaker exploded in a quite spectacular manner, and
an arc was maintained until the source was turned off. Under the same
circumstances, fuses with ACIR of 100k or 200k only exhibited slight
movement and perhaps a puff of magic smoke.

My colleague who has me design these test sets was once performing
switchgear maintenance with an older engineer, during a shutdown of a
commercial facility after hours. The engineer was using an adjustable
wrench, which did not have a properly insulated handle, to loosen some bolts
on live buswork, and the wrench slipped, falling across the main bus from
the distribution transformer, and it created a bolted short and a huge
fireball. My friend had just enough time to turn aside, and was still badly
burned, but the engineer received injuries that were eventually fatal. They
were both wearing safety glasses, but no other protective gear. I found one
video clip that really demonstrates the need for arc blast suits, and shows
what can be survived:

The company I originally worked for did breaker testing, and one of my first
jobs was at a large government facility in Newark, OH. We removed, tested,
serviced, and replaced hundreds of metal clad breakers, with no problems.
But a few years later one of our technicians was seriously injured when a
stab finger cluster came loose during racking and fell across an energized
bus. So I have great respect for what can happen when high power electricity
is involved. And it's technically still "low voltage" since it's les than
600V.

Back to the protective measures of the test sets we make, the LPS-200 fuses
http://www1.cooperbussmann.com/pdf/19842fdd-6186-4ce8-b245-16e25e01cbd4.pdf
limit the maximum available fault current to 20kA where available fault
current is 100kA. So for all practical purposes, the fusing or other
protection beyond that can have an ACIR of 20kA or even 10k, which is in the
range of most circuit breakers and fuses used for instrumentation. The
control circuitry is then fused with something like FNQ-5 fuses which limit
fault currents to less than 100 amps, and a true interrupting rating of
10kA.

But the real problem is the connection of the gate wires to the SCRs, which
are capable of several thousand amperes during normal use. The SCR trigger
boards we have used in the past have split bobbin transformers for the gate
circuitry, and opto-isolators rated at 5kV breakdown. We have not had any
problems with these boards. The phase reference has always come from the 120
VAC control circuitry, and that is safely limited and protected. Much of the
safety of these boards is built into their physical design, with large
clearances between the AC mains and the low voltage control circuitry. But
this new design will be in a smaller package, and will optionally derive the
zero crossing detection from the voltage across the SCRs, so that a 12 VDC
supply can power the board circuitry.

There are also other safeguards built into the newer test sets. After the
primary fuses, we have a pair of large contactors which are energized from
the AC control voltage, through a series of interlocks. These interlocks can
be thermal sensors, safety switches, or circuitry which monitors output
current for overload conditions. For this we have designed a Programmable
Overload Device (POD), which calculated the safe operating overloads and
shuts down as needed. We have also considered a "Ground Overload Device"
(GOD), to detect ground faults.

But, as my colleague asserts, having too many safeguards can also be a
liability, if the circuitry itself fails and renders the test set unusable.
And also there is the premise that a technician may even get careless if he
thinks there are all sorts of protective safety devices that will prevent
damage or personal injury if the test set is misused.

So, there needs to be a tradeoff and limitation on what can happen to cause
a dangerous condition, and what can be done to avoid it in the first place,
or deal with it safely and effectively if it does happen. Permanent
electrical installations are covered by many standard practices and the NEC
code, while portable test equipment is covered somewhat by NEMA standards,
but because of its nature is not as straightforward. In the case at hand, I
am trying to look at as many ways as possible to determine what could happen
that could affect safety, and what can be done, reasonably, to prevent
problems or deal with any that could happen. I think the surface mounted
500V fuses are a reasonable precaution, and I may also include them in the
gate connections. But in reality, the board should just be designed with
adequate clearances and perhaps some insulating barriers to preclude any
problems.

The most likely source of failure is the optoisolators, since they are very
small and there could be some deterioration or manufacturing defect that
could cause a breakdown. The only way to absolutely be sure is to use
separate transmitters and receivers separated by a substantial length of
light pipe. The other source of breakdown would be the transformers, and
properly designed split bobbins are very reliable. Other than that, the only
failure mechanism would be external sources of conductive material, such as
loose strands of wire or contamination from carbon or metallic deposits, and
those can be prevented by proper mounting and perhaps a protective cover.

Sorry for the long essay, but I wanted to present these points for my own
analysis as well as invite other comments. Thanks.

Paul
www.pstech-inc.com

 

[Glen Walpert]

"Glen Walpert" wrote in message







You raise some excellent points, and I am familiar with the concepts of
available fault currents, interrupting rating, and impedance factors
(also sometimes known as regulation, in %). Since you brought this up,
it will be helpful to discuss the details of these test sets and their
safety issues.

First, there is no way of knowing what sort of mains supply it will be
connected to, in the field, so we must assume the worst, which would be
probably a 480 VAC supply with a 400A or even 1200A capacity, and it is
very possible that the source may be of even higher kVA. We once took
our largest test set to an electrical generating station and connected
the input as solidly as possible to their largest service, and we were
able to obtain more than 100kA on the output into a shorting bar, with
an open circuit voltage of about 12 VAC. This was a test set rated about
6000A continuous, so a rough estimate of the impedance factor (or
regulation) was about 5%, as you said, with the output about 20x
nominal. The current draw on the mains would be about 1/40 that of the
secondary, or about 2500 amps.

The early versions of these test sets were protected with only a MCB
rated at 200 to 400 amps, with interrupting rating of about 12,000 amps.
Our newer test sets have a pair of LPS-200 fuses which are rated at
200kAIC, and that should be sufficient for most normal services which
are protected by current limiting fuses. When an extreme overload
occurs, such fuses trip in less than 1/2 cycle, even before the first
peak, so they limit the actual "let-through" current to something that a
MCB can handle, and in fact will probably stop the current before the
breaker even has time to operate. I have seen a movie comparing the
performance of fuses and MCBs with various available fault currents, and
even at the rated IC, the breaker was badly damaged, but it did its job
of clearing the fault. At much higher fault currents, however, the
breaker exploded in a quite spectacular manner, and an arc was
maintained until the source was turned off. Under the same
circumstances, fuses with ACIR of 100k or 200k only exhibited slight
movement and perhaps a puff of magic smoke.

My colleague who has me design these test sets was once performing
switchgear maintenance with an older engineer, during a shutdown of a
commercial facility after hours. The engineer was using an adjustable
wrench, which did not have a properly insulated handle, to loosen some
bolts on live buswork, and the wrench slipped, falling across the main
bus from the distribution transformer, and it created a bolted short and
a huge fireball. My friend had just enough time to turn aside, and was
still badly burned, but the engineer received injuries that were
eventually fatal. They were both wearing safety glasses, but no other
protective gear. I found one video clip that really demonstrates the
need for arc blast suits, and shows what can be survived:

The company I originally worked for did breaker testing, and one of my
first jobs was at a large government facility in Newark, OH. We removed,
tested, serviced, and replaced hundreds of metal clad breakers, with no
problems. But a few years later one of our technicians was seriously
injured when a stab finger cluster came loose during racking and fell
across an energized bus. So I have great respect for what can happen
when high power electricity is involved. And it's technically still "low
voltage" since it's les than 600V.

Back to the protective measures of the test sets we make, the LPS-200
fuses
http://www1.cooperbussmann.com/pdf/19842fdd-6186-4ce8- b245-16e25e01cbd4.pdf
limit the maximum available fault current to 20kA where available fault
current is 100kA. So for all practical purposes, the fusing or other
protection beyond that can have an ACIR of 20kA or even 10k, which is in
the range of most circuit breakers and fuses used for instrumentation.
The control circuitry is then fused with something like FNQ-5 fuses
which limit fault currents to less than 100 amps, and a true
interrupting rating of 10kA.

But the real problem is the connection of the gate wires to the SCRs,
which are capable of several thousand amperes during normal use. The SCR
trigger boards we have used in the past have split bobbin transformers
for the gate circuitry, and opto-isolators rated at 5kV breakdown. We
have not had any problems with these boards. The phase reference has
always come from the 120 VAC control circuitry, and that is safely
limited and protected. Much of the safety of these boards is built into
their physical design, with large clearances between the AC mains and
the low voltage control circuitry. But this new design will be in a
smaller package, and will optionally derive the zero crossing detection
from the voltage across the SCRs, so that a 12 VDC supply can power the
board circuitry.

There are also other safeguards built into the newer test sets. After
the primary fuses, we have a pair of large contactors which are
energized from the AC control voltage, through a series of interlocks.
These interlocks can be thermal sensors, safety switches, or circuitry
which monitors output current for overload conditions. For this we have
designed a Programmable Overload Device (POD), which calculated the safe
operating overloads and shuts down as needed. We have also considered a
"Ground Overload Device" (GOD), to detect ground faults.

But, as my colleague asserts, having too many safeguards can also be a
liability, if the circuitry itself fails and renders the test set
unusable. And also there is the premise that a technician may even get
careless if he thinks there are all sorts of protective safety devices
that will prevent damage or personal injury if the test set is misused.

So, there needs to be a tradeoff and limitation on what can happen to
cause a dangerous condition, and what can be done to avoid it in the
first place, or deal with it safely and effectively if it does happen.
Permanent electrical installations are covered by many standard
practices and the NEC code, while portable test equipment is covered
somewhat by NEMA standards, but because of its nature is not as
straightforward. In the case at hand, I am trying to look at as many
ways as possible to determine what could happen that could affect
safety, and what can be done, reasonably, to prevent problems or deal
with any that could happen. I think the surface mounted 500V fuses are a
reasonable precaution, and I may also include them in the gate
connections. But in reality, the board should just be designed with
adequate clearances and perhaps some insulating barriers to preclude any
problems.

The most likely source of failure is the optoisolators, since they are
very small and there could be some deterioration or manufacturing defect
that could cause a breakdown. The only way to absolutely be sure is to
use separate transmitters and receivers separated by a substantial
length of light pipe. The other source of breakdown would be the
transformers, and properly designed split bobbins are very reliable.
Other than that, the only failure mechanism would be external sources of
conductive material, such as loose strands of wire or contamination from
carbon or metallic deposits, and those can be prevented by proper
mounting and perhaps a protective cover.

Sorry for the long essay, but I wanted to present these points for my
own analysis as well as invite other comments. Thanks.

Paul www.pstech-inc.com

Thanks for the long essay, I think anything which increases awareness of
the hazards of high power electrical distribution is a good thing. Good
video link too. Clearly you have given this matter the serious
consideration it deserves. The only other thought I have is that it may
be desirable to design the enclosure so that if an internal arc blast
were to occur it would vent out the back, away from the operator, either
with adequate open vent area or with the back panel being designed to
blow open without detaching. I expect you have already considered this
too.

I was fortunate in that no one was ever hurt or killed on any of the jobs
I worked on involving high power electrical systems (mostly military
facility design and startup testing, ending in 1998). But people I
worked with lost 4 coworkers on other jobs; 1 low voltage electrocution,
2 from medium voltage burns and 1 arc blast death from the shock wave.
All due to human error rather than equipment failure. The arc blast
death occurred when an electrician opened an air-cooled breaker scheduled
for replacement, went to get the drawing rack, came back with it and
pulled the wrong breaker, which was under load. The scariest indecent
occurred at the New London CT submarine base, where an electrician went
to inspect a new enclosed pad mount substation installation, where the
installers neither grounded the enclosure, bolted it down. A truck then
backed into the enclosure pushing it into contact with a medium voltage
terminal. The electrician grabbed the lock to open it, and suffered
burns requiring amputation of an arm and a leg, plus such severe
neurological damage that he was still unable to talk a year after the
incident. Prior to that, I never once considered the possibility that
the enclosure of distribution equipment might be hot, and touched
numerous enclosures with my bare hand without concern.

I seems that many still have a cavalier attitude towards electrical
safety, as evidenced by an article in the November 2011 Control
Engineering magazine titled "Electrical Controls Dirty Little Secret: We
Don't Follow NFPA Rules", which basically mocks NFPA 70e safety rules.
Pity the magazine was foolish enough to publish it.

Regards,
Glen

 

[Joerg]

"Joerg" wrote in message


Welwyn also has some rated at 1000V. But Newark doesn't stock them:
http://www.welwyn-tt.com/pdf/datasheet/SPH-SPF.PDF

Their selector shows them as "level 69V" and the EMC2 as "level 11.7V".
I don't know what that means.
http://www.welwyn-tt.com/products/r...sp?application=Fusible&technology=%&package=%


Actually 500V is probably enough. 600V is more of a device class, but
some fuses like the KTK are 600V, and the same size fuses in slow blow,
FNQ, are rated 480V.

From a liability point of view they should be rated for the maximum
applied peak voltage. If something bad happens that'll be the first
thing the expert witness will be looking at, the datasheet. 480V is
nothing to sneeze at, it can cause major facial injury, loss of eyesight
and so on.

480 VAC service is usually 277 to ground, for wye connections, but it
could be delta, and a phase could be as high as 480 VAC to ground. Our
new test sets are not rated for 600V input, but older ones are. And they
typically have a tap switch that has a maximum setting of 600 VAC, even
on 208V input.

Mostly I worry about a fault from line to ground, or something like the
120 VAC control voltage. In that case, depending on phase, a 480V supply
could be 600V to ground in some cases. There is no way to protect
against all possibilities. There are also two gate/cathode pairs that
come onto the board. Maybe I need to fuse those as well.

But my customer does not believe in anything more than the most basic
protection. ...

Aha! That would be the fire extinguisher on the wall, I assume

... The earlier test sets had something like ten fuses rated
30-60A 600V, to protect a set of relays, and they have been removed. We
also have a voltage relay which does not allow the test set mains to be
energized unless the power inputs are properly configured for 208/240 or
480 VAC. But we had some problems with the VRLY that disabled the test
set and it had to be jumpered out until it could be repaired, so the
technician could use it. So now my customer sees any protective device
as a potential liability, and says he would rather have a test set
heavily damaged by an incorrect connection, than have it falsely prevent
the technician from using it.

Management vs Engineering politics. Dilbert, anyone?

Show them this

 

[P E Schoen]

"Joerg" wrote in message

From a liability point of view they should be rated for the
maximum applied peak voltage. If something bad happens
that'll be the first thing the expert witness will be looking at,
the datasheet. 480V is nothing to sneeze at, it can cause
major facial injury, loss of eyesight and so on.

Show them this

Yes, I've seen that. I recommend that we supply or require the same
protective gear that is used in this video:

My colleague has witnessed first hand what a 480 VAC arc flash can do. He
was badly burned and his associate had his face in the cubicle where he
dropped his uninsulated wrench, and received severe injuries that proved
fatal. There have been many unsafe conditions where I work for him, as a
consultant. We have a source of 480 VAC at 400A, and a sub-panel fused at
30A for testing units that we are unsure of. And even a portable unit that
boosts 120 VAC to as much as 600 VAC but only capable of about 2-3 amps
before kicking out the breaker. And also some 208 VAC sources that are
nominally 20A.

But I had hooked up a test set to one of these sources, which had three
wires coming out of the box to connect to the test set. IIRC it had two red
wires and a black wire, so I assumed the reds were two phases and the black
wire was ground. But actually one of the red wires was ground, and
identified only by having a lug rather than a clip. So the test set only had
120 VAC supplied to it, and the chassis was at 120 VAC to ground.
Fortunately nobody was injured, and I informed the management about this
problem, and eventually they used a properly identified green wire for
ground.

Sometimes the customer does not connect the ground. I found this once on a
job in Alabama where I was trying to calibrate a 50,000 amp test set and my
test equipment was acting strangely. And I also was getting tingles from the
cabinet. So I pulled it out from the wall and found that it was not
grounded, and apparently never had been, as they had to run another wire. We
tried to design a ground integrity sensor, but it did not work on all line
configurations. Later test sets use an isolated green binding post for a
separate connection to an external ground, and a low voltage current is
applied that must flow through this connection back to the main equipment
ground on the test set. But usually the customer just jumpers the two
together and uses one ground wire.

We have also attempted to design a GFCI circuit, but it is just about
impossible to detect 20 mA when the source can draw surges of over 2000 amps
in normal use!

Paul

 

[P E Schoen]

"Glen Walpert" wrote in message

Thanks for the long essay, I think anything which increases awareness
of the hazards of high power electrical distribution is a good thing.
Good video link too. Clearly you have given this matter the serious
consideration it deserves. The only other thought I have is that it
may be desirable to design the enclosure so that if an internal arc
blast were to occur it would vent out the back, away from the
operator, either with adequate open vent area or with the back
panel being designed to blow open without detaching. I expect
you have already considered this too.

Most of our earlier units were basically a 14 gauge steel box, with a bottom
frame on casters, and expanded metal to keep mice out of the interior. So
any major arc flash would expel hot metal and flames out the bottom.
Hopefully the operator is not wearing sandals! But one of our new units is
built with aluminum T-slot channels, and the side panels are merely 1/4" ABS
plastic. At least the top is a steel plate. You can see a video of these
breaker test sets here:

and others along the same line:

http://youtu.be/Xr713ZUsJws (4000A DC nominal, tested up to 20 kA)
http://youtu.be/3o9yy7zJy58 (a USB based analyzer I designed for
calibration)

And yes, I should be wearing safety glasses and other protective gear!

[snip]

I seems that many still have a cavalier attitude towards electrical
safety, as evidenced by an article in the November 2011 Control
Engineering magazine titled "Electrical Controls Dirty Little Secret:
We Don't Follow NFPA Rules", which basically mocks NFPA 70e
safety rules.
Pity the magazine was foolish enough to publish it.

Perhaps it was an example of what NOT to do.

Paul

 

[josephkk]

Thanks for the long essay, I think anything which increases awareness of
the hazards of high power electrical distribution is a good thing. Good
video link too. Clearly you have given this matter the serious
consideration it deserves. The only other thought I have is that it may
be desirable to design the enclosure so that if an internal arc blast
were to occur it would vent out the back, away from the operator, either
with adequate open vent area or with the back panel being designed to
blow open without detaching. I expect you have already considered this
too.

I was fortunate in that no one was ever hurt or killed on any of the jobs
I worked on involving high power electrical systems (mostly military
facility design and startup testing, ending in 1998). But people I
worked with lost 4 coworkers on other jobs; 1 low voltage electrocution,
2 from medium voltage burns and 1 arc blast death from the shock wave.
All due to human error rather than equipment failure. The arc blast
death occurred when an electrician opened an air-cooled breaker scheduled
for replacement, went to get the drawing rack, came back with it and
pulled the wrong breaker, which was under load. The scariest indecent
occurred at the New London CT submarine base, where an electrician went
to inspect a new enclosed pad mount substation installation, where the
installers neither grounded the enclosure, bolted it down. A truck then
backed into the enclosure pushing it into contact with a medium voltage
terminal. The electrician grabbed the lock to open it, and suffered
burns requiring amputation of an arm and a leg, plus such severe
neurological damage that he was still unable to talk a year after the
incident. Prior to that, I never once considered the possibility that
the enclosure of distribution equipment might be hot, and touched
numerous enclosures with my bare hand without concern.

I seems that many still have a cavalier attitude towards electrical
safety, as evidenced by an article in the November 2011 Control
Engineering magazine titled "Electrical Controls Dirty Little Secret: We
Don't Follow NFPA Rules", which basically mocks NFPA 70e safety rules.
Pity the magazine was foolish enough to publish it.

Regards,
Glen

Scary indeed. For the past few years my job has expanded to include arc
flash hazard issues and i have my own copy of IEEE 1584 and some other
related stuff.

?-)

 

[Cydrome Leader]

"Joerg" wrote in message



Yes, I've seen that. I recommend that we supply or require the same
protective gear that is used in this video:

My colleague has witnessed first hand what a 480 VAC arc flash can do. He
was badly burned and his associate had his face in the cubicle where he
dropped his uninsulated wrench, and received severe injuries that proved

I have to ask what- the **** is it with electricians dropping uninsulated
wrenches all the time? Nobody was hurt, but a giant power outage at a
datacenter I used was caused by guess it -a electrician dropping an
uninsulated wrench into some 480V switch gear. It's like clockwork that
this happens.

tip for electricians or anybody working on live ciruits with dangerous
amounts of energy present- use insulated tools.

 

[Cydrome Leader]

Paul,

*IF* you're serious about GFCI circuitry to detect that 20mA out of
2000A+, we should talk - I think I see a way to do it.

I'm not sure of the sensitivity, and I'm guessing it's more than 20mA but
in mining they have weird GFCI breakers which operate by injecting an
extra signal into the power and then looking for this signal in the
ground. Looking for plain current imbalances just doesn't work for this
stuff. I have no idea how they detect an electrified puddle with cables in
it over plain leakage from the 800Hz (something like that) that they
inject into the power.

 

[Glen Walpert]

"Glen Walpert" wrote in message


Most of our earlier units were basically a 14 gauge steel box, with a
bottom frame on casters, and expanded metal to keep mice out of the
interior. So any major arc flash would expel hot metal and flames out
the bottom. Hopefully the operator is not wearing sandals! But one of
our new units is built with aluminum T-slot channels, and the side
panels are merely 1/4" ABS plastic. At least the top is a steel plate.
You can see a video of these breaker test sets here:

and others along the same line:

http://youtu.be/Xr713ZUsJws (4000A DC nominal, tested up to 20 kA)
http://youtu.be/3o9yy7zJy58 (a USB based analyzer I designed for
calibration)

And yes, I should be wearing safety glasses and other protective gear!

Interesting videos. Some safety gear is no doubt appropriate, perhaps
safety glasses, face shield and hearing protectors. My feeling is that
it is a good idea to read the current safety standards and conform to
their requirements completely, mostly because those standards are very
much experience based and proved effective but also so that if there is
an injury it won't be due to negligence on my part. None of the current
standards were out when I last worked with high power distribution
equipment, so I can only guess what is now required for the fairly low
energies in your testers. I like to go a bit beyond in some cases and
duct tape rubber mats over nearby conductive material if it is for some
reason necessary to do hot work - in addition to using properly insulated
tools.

The 4000 amp frame breaker in your video is much smaller than one with
the same rating I recall well, which was a 200 kAICR ITE-Siemens
switchboard breaker designed for remote operation. Final customer
acceptance test underway, and the breaker will not operate except
manually - remove a cover from the breaker, insert a large crank, turn
about 20 times to compress the opening spring and then close the
breaker. We racked out the breaker and found that the 120 VDC control
power rectifier was open (the supply consisted of taps from the 480 input
terminals to a big cartridge fuse (no wimpy glass fuses in switchboards)
to a ~500 VA transformer to a single house numbered stud diode, no
unreliable capacitor of course, and not required for a trip). I called
the factory, gave them the serial number, and was told that it was still
under warranty so they could not sell me any parts or provide any
information, I should pack up the 1600 pound breaker in it's original
crate and send it back to the factory for repair, they would turn it
around in 120 days max. I explained that I had 20 people on site waiting
to continue a plant acceptance test and that wouldn't do, so they said if
we bought a new one they could probably get it to us in only 60 days. I
told them we would consider our options, then went to the nearest Radio
Shack and bought a diode suitably rated to protect the expensive fuse
(just like the original), resuming testing in about 2 hours. Memorable
only because it was the least useful call to a vendor I ever made.

[snip]

I seems that many still have a cavalier attitude towards electrical
safety, as evidenced by an article in the November 2011 Control
Engineering magazine titled "Electrical Controls Dirty Little Secret:
We Don't Follow NFPA Rules", which basically mocks NFPA 70e safety
rules.
Pity the magazine was foolish enough to publish it.

Perhaps it was an example of what NOT to do.

If you can't be an example, be a warning .

Glen