Has this ever happened to you ?

[goldstar]

I recently installed a temporary 3-phase 120/208 service to a retail space that was newly renovated (and waiting on the POCO to install a meter). I ran a short (20') 3-phase SER cable from an adjacent store and an existing 200 amp main lug panel. I used a 3-pole 90 amp Siemens breaker in the 200 amp panel and landed the other end of the SER on the main lugs of the new panel.

Now, I accidentally shorted the EGC to a phase conductor while changing out an exit light in the new section. The 20 amp circuit for the exit lights in the new panel didn't blow but the 3-pole 90 in the 200 amp panel did. Does anyone have an idea why the 3-pole 90 blew before the s/p 20 amp breaker did ?

Thanks in advance.

Phil

 

[LarryFine]

Does anyone have an idea why the 3-pole 90 blew before the s/p 20 amp breaker did ?

Usually, it's because the 90a breaker has a faster trip curve response than the 20a breaker.

 

[satcom]

Usually, it's because the 90a breaker has a faster trip curve response than the 20a breaker.

Also, a lot depends on where in the cycle he shorted the circuit.

 

[billsnuff]

wouldn't happen if the power was off

glad you weren't hurt.

 

[charlie b]

It is a random event. Both breakers received what I like to call "the command to open" at the same time, and either one of them could have obeyed that command before the other. As soon as one opened, the fault condition would disappear, so that the other would no longer need to trip. It is also possible that both could trip, if both reacted at the same speed.

The issue is called "selective coordination." It's all about choosing breakers (or fuses) and settpoints such that the OCPD closest to the fault trips before any upstream device. There is a relatively new requirement that applies to certain installations (see for example 700.27). If you have two breakers of the same time, and their trip settings are that close to each other (i.e., 20 amps and 90 amps), it is likely that they would not coordinate with each other.

 

[goldstar]

wouldn't happen if the power was off

glad you weren't hurt.

Thanks and you're right. The more I thought about it the more I realized that there was no need to have the power on for the 10 minutes that I needed to relocate the exit light. I should have gotten down off the ladder, walked over and shut the breaker.:roll:

 

[goldstar]

It is a random event. Both breakers received what I like to call "the command to open" at the same time, and either one of them could have obeyed that command before the other. As soon as one opened, the fault condition would disappear, so that the other would no longer need to trip. It is also possible that both could trip, if both reacted at the same speed.

The issue is called "selective coordination." It's all about choosing breakers (or fuses) and settpoints such that the OCPD closest to the fault trips before any upstream device. There is a relatively new requirement that applies to certain installations (see for example 700.27). If you have two breakers of the same time, and their trip settings are that close to each other (i.e., 20 amps and 90 amps), it is likely that they would not coordinate with each other.

Thanks Charlie. I was curious to know if you thought it had anything to do with the fact that the phase conductor shorted to the EGC as opposed to a neutral conductor even though both are tied together at the meter enclosure.

 

[TOOL_5150]

Thanks Charlie. I was curious to know if you thought it had anything to do with the fact that the phase conductor shorted to the EGC as opposed to a neutral conductor even though both are tied together at the meter enclosure.

Either to the grounded or grounding conductor, it would have done the same thing.

~Matt

 

[Cow]

Had this exact same scenario happen to another electrician in our shop. He blew the 3 pole 100 Siemens breaker feeding the subpanel when he shorted the emergency light circuit. That made one heck of a bang.:roll: The single pole 20 hadn't tripped...