Objectionable Current Tripping Residual-Current Device (i.e. whole-house GFCI)

[jjkind]

Greetings all,

I'm new to the forum, but thought that this was an excellent place to pose a question that has perplexed me for several years regarding the nuisance tripping of a residual-current device.

Here's the story:
I own an apartment in a small building in Buenos Aires, Argentina. When I moved in several years ago, my unit was equipped with a residual-current device along with a separate current protection circuit breaker (I use the past tense because I ultimately had to remove the residual-current device to stop the nuisance tripping).

To give an idea of the building, there are 4 units and service is distributed to each individual service panelboard (within the units) from a main distribution, multi-panelboard with the service disconnect, electrical meter, and main circuit breaker for each unit. Just to be clear, each unit has a circuit breaker external to the unit (located next to the service disconnect) and one inside of the unit in the unit service panelboard, along with a residual-current breaker (basically a whole-house GFCI...used in place of GFCI outlets, not something I had seen before).

I began having problems almost immediately with my residual-current breaker tripping for no apparent reason. Over the course of several days I came to realize that the nuisance tripping was coinciding with the arrival of my neighbor (unit below used as an office). For reasons that I cannot explain, he insists upon using the main circuit breaker in his service panel to shut off electricity to his unit EVERY DAY when he leaves. I left my doorway open one day when he arrived, and sure enough, just as I heard his circuit breaker "click," my residual-current breaker tripped.

I did some more experimenting over the next few days and found that my residual-current breaker would trip with a light load (watching my small TV, for instance - maybe 150W), but it would not trip when I was using a larger appliance like the washing machine.

The current threshold for the residual-current breaker is 30mA, and whenever it trips I can reset it and it holds without tripping - meaning that, for reasons that I cannot explain, the fault current is only exceeding this value in the moment when my neighbor turns on his main circuit breaker, and only when I am using a light load.

I tried to talk to local electricians about the issue - and I even called technical support with the distribution company at one point in time - but nobody has been able to offer me suggestions on potential corrective actions to take to eliminate the problem. As I mentioned, I ultimately had to take the residual-current breaker out of the system to prevent the nuisance tripping.

I've latched onto the idea that there must be some sort of objectionable current path in the electrical system, but what makes no sense to me is why or how my neighbor operating his main circuit-breaker is affecting my residual current breaker.

Has anyone seen something like this before or have an explanation?

Thanks for reading!

Regards,
Jeremy

 

[broadgage]

I would suspect that you have a high resistance fault between the neutral conductor and ground/earth.
Under normal conditions this wont pass much current, since the neutral is/should be at close to ground voltage.
When your neighbour turns off his main, this may alter the voltage between ground and neutral, and pass enough current to trip the RCD.

It would be advisable to engage an experienced local electrician to test the insulation resistance between your wiring and earth/ground, and to rectify any faults found.
Appliances involving water such as washers are a fruitfull cause of such faults.

If local regulations permit, it might be worth removing the main RCD and instead fitting each circuit with its own RCD. That should at least localise the problem.

All this presumes that the installations is supplied from a system with a grounded neutral, in line with UK or USA practice.

If the supply is from an ungrounded system, then the voltages between phase conductors and true earth will vary widely as other loads on the system, with varying leakage currents are turned on/off. This can result in an installation tripping the RCD or not, according to what loads neighbours are using.

 

[jjkind]

Thanks for your reply.

Your assumption that the system has a grounded neutral conductor is correct. My apartment is served with two wire service rated at 220V, so the neutral wire is carrying a current.

I understand you point. I need to do an insulation resistance test between the ground and neutral conductors 1) inside my apartment (to see if the high resistance fault on the inside wiring) and if I don't find anything 2) between neutral/ground conductors from my apartment panelboard to the service entrance panelboard for the building (the fault could also reside outside of the apartment, which is located on the 3rd floor).

Here's what I don't understand.

The only point where my apartment is electrically connected to my neighbor's apartment is at the service entrance, where the neutral busbar should be bonded to the system ground. If there is indeed a high resistance fault between neutral and ground on my circuit, which is probably the case, my RCD should trip when a voltage exists between these conductors that causes at least 30mA of current to flow into the ground conductor, bypassing and thereby activing the RCD. The only thing that I can think of that would do this is a temporary increase in the impedance of the neutral wire, which would cause the aforementioned voltage increase and current to flow to the ground conductor.

What I don't understand is how my neighbor activating his circuit breaker is effectively increasing the impedance of my neutral conductor? Is my understanding of the problem correct here? I'm worried that there is a second problem hiding, beyond the high resistance fault that you mentioned.

Any comments you have on the subject would be appreciated.

 

[mbrooke]

Can you post a picture of the insides of your panelboard? It would help me understand your system better.

It is possible that you may have a crossed neutral or hot with one of your neighbors on a circuit. During construction an electrician may have accidentally run a line from one of your power sockets to one in your neighbors thinking it came from the right place. Does your unit share any walls or floors with your neighbor in question?

Another possibility is that the RCD may have been simply defective to begin with. A new one may not do the same.

 

[jjkind]

Thanks for the response, but unfortunately I don't have any photos to post.

I've considered that the RCD may be faulty, and I agree that it would be a prudent step to purchase another. I may give that a try, in the case that I cannot find any high resistance faults in my wiring (that's still the #1 suspect for me).

My unit is above his, and we only share a floor. Given the construction - poured concrete floor and structure with block walls - I highly doubt the possibility that any neutral/ground lines are crossed between the units.