Lightning tripping Main

[dms292]

I have a panel that serves outdoor lighting poles as well other indoor loads. A lightning strike on one of these poles took out the main of this panel. They previously prevented this problem in another building by installing a sub panel that has a main breaker with a ground trip setting. The idea is that a lighting strike will take out the sub panel main, and not take the remainder of the other panel down as well.
My question for our application is what does the rating of the sub panel need to be. If based solely on fault current, a 18k breaker is sufficient, however the previous solution called for a 200k panel (series rated I assume). I can find nobody that knows why such a large rating was specified (the previous engineer is gone). I believe it is over rated, however, I cannot believe he specified that on a whim. What am I missing?

 

[steve66]

The utility may only be able to provide 18KA, but how much current can a lighting strike provide?

I think the subpanel main will have to open during the strike, possibly with the full lightning strike current flowing through it.

 

[big john]

I may be about to learn something, but I have never heard of anyone installing a "sacrificial breaker" to attempt to limit damage from a lightning strike.

I would argue that by the time there is enough current flow to operate a breaker, there may already be very significant overvoltages on the conductors. And regardless of whether the breaker is capable of interrupting that surge, the damage will likely already be done.

I think a better solution would be a combination of lightning arrestors and surge protection devices working in tandem.

 

[dms292]

I may be about to learn something, but I have never heard of anyone installing a "sacrificial breaker" to attempt to limit damage from a lightning strike.

I would argue that by the time there is enough current flow to operate a breaker, there may already be very significant overvoltages on the conductors. And regardless of whether the breaker is capable of interrupting that surge, the damage will likely already be done.

I think a better solution would be a combination of lightning arrestors and surge protection devices working in tandem.

The new panel main breaker is not actually sacrificial. There is no damage to the existing system. They found that lightning strikes were tripping the main breaker of the panel, so they installed a sub panel with ground fault protection on it so that any lightning trips out that panel rather than the main panel. The main panel has some loads on that are critical to the process, and they do not want a parking lot lighting issue tripping the process. As to why the lightning was tripping the main and not the feeder breakers, I am not sure. Lightning is a finicky beast.

 

[gadfly56]

The new panel main breaker is not actually sacrificial. There is no damage to the existing system. They found that lightning strikes were tripping the main breaker of the panel, so they installed a sub panel with ground fault protection on it so that any lightning trips out that panel rather than the main panel. The main panel has some loads on that are critical to the process, and they do not want a parking lot lighting issue tripping the process. As to why the lightning was tripping the main and not the feeder breakers, I am not sure. Lightning is a finicky beast.

How about a separate sub-panel for the parking lot lights?

 

[big john]

The new panel main breaker is not actually sacrificial. There is no damage to the existing system. They found that lightning strikes were tripping the main breaker of the panel, so they installed a sub panel with ground fault protection on it so that any lightning trips out that panel rather than the main panel....

Gotcha. Has anyone investigated the grounding at your facility?

If your system is grounded at both the switchgear and the transformer, I can envision a scenario where earth-current from a lightning strike might end up flowing on your neutral conductor and this would definitely mess up the main GFP. While I admit I've never seen the problem you describe, it's very common for me to find the bonding jumper installed in both the transformer and in the main switchgear.

It would also explain why you don't appear to have any other equipment damage, because lightning wasn't directly damaging conductor insulation to get onto the phase conductors.

 

[mopowr steve]

Gotcha. Has anyone investigated the grounding at your facility?

If your system is grounded at both the switchgear and the transformer, I can envision a scenario where earth-current from a lightning strike might end up flowing on your neutral conductor and this would definitely mess up the main GFP. While I admit I've never seen the problem you describe, it's very common for me to find the bonding jumper installed in both the transformer and in the main switchgear.

It would also explain why you don't appear to have any other equipment damage, because lightning wasn't directly damaging conductor insulation to get onto the phase conductors.

I would have to concur, are you sure that underground conductors are not faulting after a downpour ?. As the soil moisture level rises the resistance between the conductors goes down raising current levels.
Otherwise grounding may be the issue.