Intermittent Voltage Drop

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ggm

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I have a situation where a 200 amp main breaker panel is installed in an outbuilding which then feeds a 100 amp panel in the residence (this was done to upgrade to 200 amp service when the outbuilding was built). The auxiliary panel in the residence still has the 100 amp main breaker, but the ground and neutral were isolated. This was done 8 years ago and there have been no problems observed until now.

Here is what happened: The 200 amp main breaker and the 100 amp breaker in the outbuilding went out simultaneously when the homeowner manually turned off the main breaker to add a recep to an existing circuit and then turned the breaker back on. I considered it alarming that both breakers would go bad simultaneously, but we replaced them both and then started looking for the cause of the problem. Everything seems to be fine in the outbuilding. There is constant 120 volts at the lugs in the residence where the 100 amp panel is fed. There is tight connection of these conductors as well. However, at the terminals of the main breaker in the residence, there is intermittent voltage drop - enough to cause the lights to dim/flicker. Turning on one breakers one at a time and/or testing one circuit at a time only seems to decrease the frequency of the occurrence of the voltage drop but will not make it go away. It is apparent that the problem is either internally in the main breaker of the auxiliary panel or in the connection of the main breaker to the buss. The main breaker is not bolt-on, and the 100 amp panel is packed full including several tandem breakers, so I recommended replacing the entire panel with a new 100 amp panel with more circuit capacity. This would seem sure to fix the apparent problem of the intermittent voltage drop.

But, what really concerns me and has me perplexed is why would 3 breakers go "bad" at once...and where did the problem start...and, if it started in the auxiliary panel, why would it affect the other two breakers upstream. I don't want to swap the 100 amp panels and just put a bandaid on the problem only to have it arise again later on.

Any suggestions?
 
Did the three breakers really "go bad," or did they simply move to their "trip free" positions? Did you try to reset the breakers, or was there some physical evidence that caused you to believe that they had suffered damage during the event?

It is not at all unusual, especially in a residential application, for a short circuit at a small branch circuit to cause the branch circuit breaker and the main breaker to both trip. In your case, as you have two panels each with a main breaker, I would not be at all surprised to see them both trip at the same time. Why should they not? They both saw the same amount of fault current, so their internal mechanisms would both simultaneously begin the process of tripping the breaker. If one of them acts much faster than the other, then perhaps it will trip and the other will remain closed. But there is no way to be confident in that result without performing a selective coordination study. But here again, in most homes, I would expect that you cannot achieve selective coordination with the breaker types and ratings typically in place.
 
ggm said:
However, at the terminals of the main breaker in the residence, there is intermittent voltage drop - enough to cause the lights to dim/flicker.
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I am not sure I understand what you are trying to describe here. Unless you had a voltmeter attached to the terminals of the breaker, and watched as the measured voltage took a dive and then recovered, then I don't know what you mean by "intermittent voltage drop." Are you simply saying that the lights flicker, or did you actually watch the voltage dip and recover on a voltmeter?

 
charlie b said:
Did the three breakers really "go bad," or did they simply move to their "trip free" positions? Did you try to reset the breakers, or was there some physical evidence that caused you to believe that they had suffered damage during the event?

It is not at all unusual, especially in a residential application, for a short circuit at a small branch circuit to cause the branch circuit breaker and the main breaker to both trip. In your case, as you have two panels each with a main breaker, I would not be at all surprised to see them both trip at the same time. Why should they not? They both saw the same amount of fault current, so their internal mechanisms would both simultaneously begin the process of tripping the breaker. If one of them acts much faster than the other, then perhaps it will trip and the other will remain closed. But there is no way to be confident in that result without performing a selective coordination study. But here again, in most homes, I would expect that you cannot achieve selective coordination with the breaker types and ratings typically in place.
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When the breakers were removed and tested there was not continuity on one or both poles. Resetting the breakers several times had no affect.
 
charlie b said:
I am not sure I understand what you are trying to describe here. Unless you had a voltmeter attached to the terminals of the breaker, and watched as the measured voltage took a dive and then recovered, then I don't know what you mean by "intermittent voltage drop." Are you simply saying that the lights flicker, or did you actually watch the voltage dip and recover on a voltmeter?

Click to expand...

You understand perfectly, I held a voltmeter on the terminals and watched the voltmeter dip and recover.
 
When a system that was working suddenly has a problem, it is prudent to look first at whatever was recently done to the system. My suspicion is that the homeowner made a serious wiring error, when installing the receptacle. That caused the short circuit and the tripping of all three breakers. It likely also caused the damage to the breakers. I would not be surprised to learn that the homeowner realized his mistake, and fixed the wiring error before calling you to investigate the system. In any event, the broken breakers (is that a pun? :happyyes: If so it is a bad one. :ashamed1:) are likely attributable to the installation of the new receptacle.

But I gather that the intermittent voltage drop is also a new symptom, one that did not show itself before the breaker trip event. I would first confirm this with the homeowner. If that is the case, then it remains to discover how the short circuit event could cause the intermittent voltage drop symptom. My best guess is that a wire was somewhat loose, but not loose enough to cause the symptom. A large short circuit current can cause wires to shake violently within their conduits (a result of high values of attractive and repulsive magnetic fields). That could cause a loose wire to become looser. So I would check the tightness of all connections in both panels.

If the intermittent voltage drop symptom was in evidence before the short circuit event, then I would try to correlate the occurrences with the starting of any large motors, such as an air conditioning compressor.
 
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