Here's a puzzle (well, it is for me).
I was working in the garage today, installing a new power point and light. I turned the circuits "off" by flipping the circuit breaker on the appropriate circuit. At some point, a couple of the wires connected to the power box must have touched, and they tripped the circuit breaker for the whole house. First I knew about it was when SWMBO wandered out to ask me what I was up to, there was no flash or any pyrotechnics. Having reset it I was somewhat puzzled, as my meter shows no power, as measured by AC voltage, between any two of the three wires (live, earth, neutral). I decided not to try the wet finger test ;-)
I have been scratching my head trying to work out why the circuit breaker tripped. Any ideas from the experts out there?
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May not answer your question, but our circuit breakers are very sensitive (new ish house) we only have to have a bulb go and it trips
Is it possible that its just coincidence?
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>> I have been scratching my head trying to work out why the circuit breaker tripped.
>> Any ideas from the experts out there?
The circuit breaker you isolated the circuit with was probably only single pole and only isolated the positive circuit, there is floating voltage and current on the neutral side which you shorted to the other wires, and the RCD detected and tripped.
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Not sure what you mean by "floating voltage", but shouldn't the meter have detected something between neutral and earth?
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Depends on your meter, and what you were trying to measure.. Neutral should be bonded to earth somewhere, but thats outside your house. In the mean time any other circuit and there fore appliance in the house could be doing anything on the neutral.
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I was trying to measure AC voltage across the neutral and earth, which I tried across all combinations of 2 wires......
I suppose the real question is, could I have got a shock given that I had only cut the power to the live, and that there was some juice still loos in the hoos? Or circuit in this case.
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>> I was trying to measure AC voltage across the neutral and earth, which I tried
>> across all combinations of 2 wires......
=
Try AC current next time, it will have shown enough milliamps to trip the main RCD,
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The coming together of neutral and earth can trip the main circuit breaker even if you've isolated the circuit you're working on by switching off the MBC/pulling the fuse as all you've done is disconnect the live feed, but left the neutral and earth are still connected.
Remember, it's called alternating current (AC) for a reason as the power alternates between the live and neutral feed. Isolating the live feed (the one that bites if you touch it) and the neutral feed is then safe.
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Hmm, OK. Guess I'll just have to make sure they don't have any sort of coming together next time I'm working on the electrics. Thanks guys.
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No, you make sure they are isolated next time, you can get a shock off the Neutral under certain circumstances.
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>> No, you make sure they are isolated next time, you can get a shock off
>> the Neutral under certain circumstances.
>>
and it gets even better if some clown has reversed the L&N between the incoming supply and the consumer unit!
I suggest that given a limited understanding of the theory, the OP should refrain from doing his own electrics.
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>> I suggest that given a limited understanding of the theory, the OP should refrain from
>> doing his own electrics.
>>
The OP would love to, but given the apparently simple nature of the intention, it seems pointless to pay someone to do it. However, lessons have been learnt!
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>> Not sure what you mean by "floating voltage", but shouldn't the meter have detected something
>> between neutral and earth?
>>
In theory there should be no potential difference between earth and neutral but neutral is at power station earth level and your domestic earth is at your local earth potential and there is/can be a difference. This is not normally enough to show on a meter to any great degree but can be enough to trip an RCD if the local earth is 'splashed' on to the neutral, an RCD by its design trips very quickly at a sudden change of current and this 'splash' can be enough; certainly is for my RCD....
This earth potential difference becomes much more critical if you are very close to, or within, a sub station or power station even to the level that telephone cables going into the stations have to be fitted with isolating links which prevent any accidental physical copper connection from the telephone conductors to any metalwork within the stations as one wire in a telephone circuit is at the local exchange earth potential not the station potential. Without going into all of the theory and boring you to death with AC theory and DC/AC conflicts it is something that both the telecoms companies and the electricity generating/transmission companies take very seriously and these days they don't spend any money without good reason!
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>> In theory there should be no potential difference between earth and neutral but neutral is
>> at power station earth level and your domestic earth is at your local earth potential
>> and there is/can be a difference.
>>
We have a transformer on a grid pole in the field and two wires enter our property at big insulator things on the corner of the house. One of the wires is plastic covered and is obviously the live .
The other wire divides into two at that point, and one feeds the neutral and the other becomes earth. There is no local earth.
I'm presuming this is an antiquated set up?
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>> We have a transformer on a grid pole in the field and two wires enter
>> our property at big insulator things on the corner of the house. One of the
>> wires is plastic covered and is obviously the live .
>> The other wire divides into two at that point, and one feeds the neutral and
>> the other becomes earth. There is no local earth.
>>
>> I'm presuming this is an antiquated set up?
No its not antiquated. Its being earthed at the transfomer (more or less) as it should, its just that your transformer is on a pole near your house, rather than the normal small locked compound at the end of the street.
I would guess you have a transformer because you are a long way away from a substation, and your grid pole is carrying more than 240 volts. How many wires you got supplying the transformer?
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>>
How many wires
>> you got supplying the transformer?
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Two - one from each of the high voltage (500 volts?) grid lines.
If it is earthed at the transformer that must mean that neutral is the same as earth?
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at the point they are joined, yes. You cant rely on that being fully the case through the rest of the house tho.
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My point is that there isn't an earth. anywhere. The house neutral and earth are the same thing, both being fed from the junction point outside where the incoming wire divides.
There is no house earthing point.
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>> There is no house earthing point.
Is that universally true? There are several systems in use inthe UK.
www.rsgb.org/emc/docs/pdf/leaflets/emc-leaflet-07.pdf (rsgb document discussing earthing from perspective of radio amateur)
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>>>> There is no house earthing point.
>> Is that universally true? There are several systems in use inthe UK.
Which is why I've kept out of this so far.... IMHO:
Perhaps the simplest way to consider the original question is to consider why the RCD trips, and progress from there.
If there is an imbalance in the current drawn through live and neutral at the "fuse box" in the house bigger than 0.03 Amp the RCD in the box will trip.
All the current drawn by all the appliances in the house (with the possible exception of a freezer) goes through the RCD, and makes its way from the RCD to the point where earth and neutral are bonded.
If there is something in the house drawing current, when you bridge neutral and earth you reduce the current in the neutral cable as you have two cables (earth and neutral) in parallel - some of the current goes down the earth lead, and this imbalance causes the RCD trip.
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Interesting article.
Ours is none of those. It's most like T-T only without the earth at the customer's premises.
We don't have RCDs, just fuse wire in traditional holders.
Last edited by: Cliff Pope on Tue 22 Nov 11 at 11:23
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Circuit breaker tripped but why? - mikeyb
