480/277 to a 208/120 panel

[steez]

Just wanted to share/get insight on something I saw recently during an arc flash study.

A small 40A 208/120V panel was improperly installed and is receiving 480/277V (panel had 208V breakers as well Square D QOBs). Existing facility which has been in use for awhile with no known issues.

My only guess is that since it is small and only feeding a nearby step down XFMR is that there just haven't been enough ( or any) faults to cause a failure. I'd imagine after the first or second fault the 208 breakers in the panel would fail.

Any insights or similar experiences, please share.

 

[petersonra]

Design standards for most equipment are pretty conservative. I am not at all surprised that it didn't explode just because it was used at a voltage it wasn't rated at.

 

[charlie b]

Voltage ratings are based on the ability of the component's insulation system to prevent leakage current from its energized metal parts to nearby not-supposed-to-be-energized metal. The interior of a panelboard has a lot of metal parts but not so much insulating material. I am not surprised there have been no faults.,

 

[ggunn]

Voltage ratings are based on the ability of the component's insulation system to prevent leakage current from its energized metal parts to nearby not-supposed-to-be-energized metal. The interior of a panelboard has a lot of metal parts but not so much insulating material. I am not surprised there have been no faults.,

I know of a situation where in the field an electrician changed out the line side single conductor phase lugs in a 480V panel for three conductor lugs, which left very little clearance between the lugs and the interior of the enclosure. I don't know how much clearance there was left because I only saw the panel after the fire. I also saw video of the arc flash because someone on site saw the smoke escaping the system in time to get his phone out and capture it; it was impressive. Fortunately, the fire was contained to the switchgear and no one was near it when it blew up.

 

[electrofelon]

I just mentioned in the color coding thread how I saw a guy feeding a load center with a 480/277. I never went back there so I can't provide any updates. I would speculate that during normal operation it's fine, but it's while clearing a fault that things could go awry. The AIC of a 240 breaker used with 480 has got to be real low. Consider that, for example, a Siemens BQD which is their common 480/277 breaker has an AIC of 14K @ 480/277, but 65K @ 240.

 

[winnie]

Below 600V most insulation is the same. Sometimes you see 300V flexible cord, but often the cord caps are rated 600V, the building wire is 600V, etc.

Where there is a practical difference is in terminal clearance (air spacing), fault clearing ability and switch opening ability. If people weren't flipping breakers and breakers weren't tripping? The fact that it didn't blow up isn't surprising.

But if someone had tried opening a loaded circuit you might have been doing a forensic analysis.

Jon