Are 240V. fused dis-connects safe and legal? (North America System)

[Beachcomber]

My air-conditioner compressor stopped working the other day. It's a
240 V. 20 Amp Circuit with a (20 amp - dual pole) fused disconnect
outdoors at the unit and a dual=pole ganged 20 Amp circuit breaker
further upstream at the service entrance.

The AC tech found one of the 20 Amp fuses at the disconnect had
weakened and (after 7 years) had just opened up due to the inrush
current on the first attempt at operation for the season.

This got me to thinking. If someone less-experienced had opened up
the AC Compressor unit without removing the local disconnect, they
could have gotten a nasty shock (or worse) from the good fuse on the
other leg if they were grounded. (This is in the USA).

So I conclude that circuit breakers are safer than fuses (because the
poles are ganged). But wouldn't it be safer to have a pull out
disconnect at the compressor without the fuses, relying on the service
panel circuit breakers for overcurrent protection? Anyone know what
the code says on this?

I remember as a kid, all my dad's apartment buildings had dual-pole
fused disconnects for the 240 V. loads. I wonder if these were safe?

Beachcomber

 

[John Ray]

This got me to thinking. If someone less-experienced had opened up
the AC Compressor unit without removing the local disconnect, they
could have gotten a nasty shock (or worse) from the good fuse on the
other leg if they were grounded. (This is in the USA).

So I conclude that circuit breakers are safer than fuses (because the
poles are ganged). But wouldn't it be safer to have a pull out
disconnect at the compressor without the fuses, relying on the service
panel circuit breakers for overcurrent protection? Anyone know what
the code says on this?

I remember as a kid, all my dad's apartment buildings had dual-pole
fused disconnects for the 240 V. loads. I wonder if these were safe?

Beachcomber

The fuses are redundant to the breaker in your case. The same concern about
someone not removing the disconnect could be said of a non-fused type. The
whole point of a local disconnect is so that you can visually verify that
power is disconnected. There's no need for the fuses in your case, but
theres no harm there.

Breakers wear out, too btw.

John

 

[operator jay]

This got me to thinking. If someone less-experienced had opened up
the AC Compressor unit without removing the local disconnect, they
could have gotten a nasty shock (or worse) from the good fuse on the
other leg if they were grounded. (This is in the USA).

This is one reason why they say trained people should work on electrical
equipment. There are far fewer mishaps that way. Electricity can seem like
a sneaky sneaky beast some times. For one thing you can't see it. LOTS of
people have gotten surprise shocks from equipment they
thought/figured/assumed was "dead".

So I conclude that circuit breakers are safer than fuses (because the
poles are ganged). But wouldn't it be safer to have a pull out
disconnect at the compressor without the fuses, relying on the service
panel circuit breakers for overcurrent protection? Anyone know what
the code says on this?

Breakers may be "safer" if lay persons are poking around into electrics.
Fortunately 120V usually just 'hurts' and only occasionally kills people or
burns down their buildings as a result.

The Code, here, calls for a disconnect near to the equipment and visible
from the equipment. Or, failing that, a padlockable disconnect. There are
many places where an electrical worker would get in serious trouble if found
working on equipment without opening the disconnect or taking even greater
measures (lock out - tag out).

j

 

[SQLit]

My air-conditioner compressor stopped working the other day. It's a
240 V. 20 Amp Circuit with a (20 amp - dual pole) fused disconnect
outdoors at the unit and a dual=pole ganged 20 Amp circuit breaker
further upstream at the service entrance.

The AC tech found one of the 20 Amp fuses at the disconnect had
weakened and (after 7 years) had just opened up due to the inrush
current on the first attempt at operation for the season.

This got me to thinking. If someone less-experienced had opened up
the AC Compressor unit without removing the local disconnect, they
could have gotten a nasty shock (or worse) from the good fuse on the
other leg if they were grounded. (This is in the USA).

So I conclude that circuit breakers are safer than fuses (because the
poles are ganged). But wouldn't it be safer to have a pull out
disconnect at the compressor without the fuses, relying on the service
panel circuit breakers for overcurrent protection? Anyone know what
the code says on this?

I remember as a kid, all my dad's apartment buildings had dual-pole
fused disconnects for the 240 V. loads. I wonder if these were safe?

Beachcomber

Older residential installations used disconnects that allowed access to one
fuse at a time. Newer installations have "pull out disconnects" Pull the
handle and you pull both fuses.

If the person is less experienced then they should not be "playing with
electricity" though a lot of people do.

Fuses are usually installed to protect the motor not the wiring. The circuit
breaker in the panel protects the wiring to the disconnect. My last house
had a pull out disconnect. Mostly because it was cheap. A lot depends on
the load and the contractor if you get a fused disconnect. My new a/c is fed
with a 50 amp breaker, the unit draws 16 amps, the disconnect has 20 amp
time delay fuses per the manufactures requirements.

 

[operator jay]

|
| |
|> This got me to thinking. If someone less-experienced had opened up
|> the AC Compressor unit without removing the local disconnect, they
|> could have gotten a nasty shock (or worse) from the good fuse on the
|> other leg if they were grounded. (This is in the USA).
|
| This is one reason why they say trained people should work on electrical
| equipment. There are far fewer mishaps that way. Electricity can seem like
| a sneaky sneaky beast some times. For one thing you can't see it. LOTS of
| people have gotten surprise shocks from equipment they
| thought/figured/assumed was "dead".
|
|> So I conclude that circuit breakers are safer than fuses (because the
|> poles are ganged). But wouldn't it be safer to have a pull out
|> disconnect at the compressor without the fuses, relying on the service
|> panel circuit breakers for overcurrent protection? Anyone know what
|> the code says on this?
|
| Breakers may be "safer" if lay persons are poking around into electrics.
| Fortunately 120V usually just 'hurts' and only occasionally kills people or
| burns down their buildings as a result.

And 277V _really_ hurts (recalling the exposed voltage that was present on
the "open door" button for over 2 weeks in an elevator in a college dorm
I once lived in).

Was that their fitness incentive program?

 

[Takoma Park Volunteer Fire Department Postmaster]

|
| |
|> This got me to thinking. If someone less-experienced had opened up
|> the AC Compressor unit without removing the local disconnect, they
|> could have gotten a nasty shock (or worse) from the good fuse on the
|> other leg if they were grounded. (This is in the USA).
|
| This is one reason why they say trained people should work on electrical
| equipment. There are far fewer mishaps that way. Electricity can seem like
| a sneaky sneaky beast some times. For one thing you can't see it. LOTS of
| people have gotten surprise shocks from equipment they
| thought/figured/assumed was "dead".
|
|> So I conclude that circuit breakers are safer than fuses (because the
|> poles are ganged). But wouldn't it be safer to have a pull out
|> disconnect at the compressor without the fuses, relying on the service
|> panel circuit breakers for overcurrent protection? Anyone know what
|> the code says on this?
|
| Breakers may be "safer" if lay persons are poking around into electrics.
| Fortunately 120V usually just 'hurts' and only occasionally kills people or
| burns down their buildings as a result.

And 277V _really_ hurts (recalling the exposed voltage that was present on
the "open door" button for over 2 weeks in an elevator in a college dorm
I once lived in).

It is terribly unlikely that there was 277 volt AC being used for
control power in an elevator. Older elevators use 600 volts DC. Newer
ones may use 480/277 for traction power but not for controls.

 

[Roy Q.T.]

I believe it is required by the NEC to have a disconnect by the
offset/outdoors AC Unit., be it a fuse or disconnect box., I've seen
those fused pull box setups around here and they are legal and
safe,(though just the disconnect is fine with the overcurrent protection
at the panel)
This way you don't have to fiddle around inside looking for and turning
the breaker at the inside panel off. It assures you (the ac service
tech) have control of the Supply Line on the Spot. ®oy