Low voltage lighting limits

[Albright's Accessories & Light]

24 volt lighting has appeared in the last couple of years which eliminates
some of the limitations of 12 volt systems--current load, voltage drop,etc.
What is the highest voltage possible without incurring a shock hazard? Does
UL have a say in this?

Jim Miller

 

[Ken]

24 volt lighting has appeared in the last couple of years which eliminates
some of the limitations of 12 volt systems--current load, voltage drop,etc.
What is the highest voltage possible without incurring a shock hazard?
Does UL have a say in this?

About 50 volts.

 

[Victor Roberts]

24 volt lighting has appeared in the last couple of years which eliminates
some of the limitations of 12 volt systems--current load, voltage drop,etc.
What is the highest voltage possible without incurring a shock hazard? Does
UL have a say in this?

I believe the voltage limit is in the National Electrical Code. I
don't have a copy here but I think the limit is about 45 to 48 volts,
close to the 50 volts stated by Ken.

 

[Andrew Gabriel]

I believe the voltage limit is in the National Electrical Code. I
don't have a copy here but I think the limit is about 45 to 48 volts,
close to the 50 volts stated by Ken.

In the UK, it's 12V near a bath or shower, and the 12V supply
must be isolated (not grounded, and generated using a power
supply which has an extra high level of isolation protection
from the mains).

There was a case of a Milkman being electrocuted by his milk
float a number of years back, and these use a 50V vehicle
battery. He went to move the milk float forwards without
actually bothering to get in the cab (which is not uncommon
when moving such vehicles only a few feet, but is of course
strictly forbidden). He misjudged it and ended up trapped
underneath, jammed against the battery terminals. He may
have been wet too, but by the time they got him out, he
had been electrocuted.

 

[Clive Mitchell]

He misjudged it and ended up trapped
underneath, jammed against the battery terminals. He may
have been wet too, but by the time they got him out, he
had been electrocuted.

If I remember correctly, his death was caused by large area cell damage
from electroporosis. This is when a DC voltage applied to the skin
causes an electrolytic action that punctures the skin cells. This in
turn gives access to the juicy interior of the body, which passes
current easily, then suffers the same fate in a larger fashion.

If you lick you arm and apply a PP3 9V battery gently to the surface,
then after several minutes you may feel a slight stinging sensation.
There will be a small red spot under where the negative (I think)
terminal was, and if you then rub that, the skin will peel slightly to
reveal a small indentation.

I was doing some experimentation with some of the quack zapper devices,
and discovered this effect. I couldn't believe such a low voltage and
current could be so destructive, but a couple more goes under scientific
conditions revealed the same results.

The arrangement I was using was a 9V battery with fly-leads soldered
onto two pennies as electrodes.

 

[Adam Aglionby]

In the UK the IEE sets the rules, extra low voltage is 50 V A.C. / 120V D.C.
:

http://www.tlc-direct.co.uk/Book/7.16.1.htm

Believe car makers want 24V so that can half their copper wire cost ;-).
Low voltage dosen`t equal low current, conductor sizes can get large
quickly.

Adam

 

[Ken]

Believe car makers want 24V so that can half their copper wire cost ;-).

No, 36V system is the future for cars (not 24V, not 48V).
When charging the voltage is 41-45 volts.
It's coming soon.

 

[jriegle]

As I have understood it, the system will basically remain 12 volts. 42 volts
will be used mainly for the formerly belt driven accessories such as the AC
compressor. These accessories can be relocated lower and back by the
firewall to distribute weight better. Some space can be saved without all of
those bolt on accessories hanging off the engine block. Some weight will be
saved due to the elimination of the thick metal brackets cast into the
engine block or bolted onto it for the belt driven accessories. One design
had the alternator in line with the crank shaft of the engine.

I've heard about the 42v system for 5 or six years now but I haven't seen
anything come about yet.
John

 

[Clive Mitchell]

Finally contact resistance is the other main ingredient for successful
electrocution. We had an electrician whose skin was so dry he could
work directly on 240V wiring. The problem is that even low levels of
current flowing through the skin open pores and, in time, stimulate
sweating creating positive feedback. Sadly, the poor milkman had
more than enough time...

The problem with your electrician who's skin resistance was high, is
that if he got complacent and handled live connections routinely, then
as soon as a wire poked through his skin or the skins resistance broke
down, he'd be in line for a bit of a wake up call.

Generally older electricians with work hardened hands and a pipe smoking
habit tend to have pretty tough high resistance skin.

 

[[email protected]]

I believe the voltage limit is in the National Electrical Code. I
don't have a copy here but I think the limit is about 45 to 48 volts,
close to the 50 volts stated by Ken.

Article 411 on interior low-voltage lighting in the 2002 NEC limits Class
requires a listed system with an isolating power supply that must not
exceed 30V (42.4 VPK) under any load condition, including open circuit. The
secondary circuits must have current limitation such that the current
cannot exceed 25A. The neutral/negative must not be grounded.

The requirements for outdoor low-voltage lighting are different than for
indoor lighting.

UL is writing a new standard UL2108 to address low-voltage lighting.

http://www.ul.com/regulators/ode/0701.pdf
http://ulstandardsinfonet.ul.com/sot/b2108_1_20030509_sum.html

HTH ... Marc
Marc_F_Hult
www.ECOntrol.org

 

[Amelia M. Samples]

FWIW, I have a TENS unit...and with the caveat that the electrodes are
approximately 2.5 cm^2 and coated with highly conductive adhesive, I can
feel the pulses at about 8V. (I put a scope probe under each electrode for
kicks) The specs say a maximum of 60V out...it's in short pulses, but OUCH!
(I tried it once)...and I've been bit by everything from 120V wall current
to a 25KV UHP lamp igniter before.