Electrical wiring question

[Fat Tone]

I have just replaced a light fitting on our scout hut. The fitting haslive, neutral and earth wires. Before starting I switched off the light switch and the circuit in the distribution board.

During fitting the Neutral wire touched the Earth wire and tripped the main breaker for the whole hut.

The wiring was professionally installed and inspected 18 months ago.

Why would that happen?

Cheers

Tony.

[dZeus]

I suppose there's always some (yet small) potential difference between neutral and earth.

When the wires touch, the residual-current circuit breaker detects the leakage and breaks the circuit.

If you have a multimeter, try measuring the potential between the neutral wire, and say, the water mains or central heating tubes.

[Umfriend]

I know little to nothing of these things but... Aren't the breakers breaking Live only? Neutral, AFAIK, need not be grouped but all go through the main breaker. Earth does not even go through the breaker so it stands to reason that where neutral is used to dissipate current from all other groups, it may have current vis-a-vis earth.

[LvR]

Both the light switch and the breaker usually only breaks the live wire by convention .................. and since modern earth leakage units detect both neutral and live leaks to earth, its doing it job protecting you.

Question is why is the neutral causing any current to flow to earth - guess faulty installation or condition of equipment somewhere in the house. Get that "professional" to came back and see what's wrong ................... could be eg a leaky heater element or even a poor connection somewhere.

[gt40]

Not sure about Uk, but in NA, only the live wire will trip a breaker.

Possibe you have a ground fault circuit interupter type breaker. they will trip if there is even a slight draw (3 ma) from live to ground. Given that it should still only trip if: the light switch was on, and the bulb was still in the socket (giving electricity a resistive path from neutral to live).
Or... there's a problem with your wiring.

[Brian Ellis]

Fairly simple. The live is one phase of a 3-phase system. The neutral is earthed at the distribution transformer. Your earth is local. Imagine that the load is equal on all 3 phases. No current will flow through the neutral wire back to the transformer, so the neutral into the building will be strictly at earth potential and touching N and E will cause no current in the E wire, assuming there are no potential differences in the ground between the two earthing points.

Now imagine that one phase has a slightly higher load than the other two. This means that the neutral will carry the unbalance current back to the transformer. This wire has a finite resistance, so there will be voltage drop across it. As it is at earth potential at the trannie, it means there will be a small voltage on the N where you are, usually <5-10 V. Touching N to E will therefore cause a small current to flow through E.

In practice, the 3 phases are never balanced, so there is always a potential on the N, generally higher the farther you are from the trannie (higher resistance of the N wiring). The electricity authorities simply hate E-N connections because it means that the E potential is different at the consumer and the trannie and a current will flow through the earth. This can cause electrolytic corrosion of nearby metal objects such as water pipes, because, as likely as not, the return earth current will flow from your earthing point to the pipe and from the pipe, 500 m down the road, to the trannie earthing point. For safety's sake, the main breaker by the meter has an earth current detector (ELCB Earth Leakage Current Breaker) and, if the leakage current exceeds a preset value, often 30 mA, it will cut both the phase and the neutral. This will eliminate a risk of fire if there is a deterioration in the insulation of an earthed appliance, such as a cooker. Yt never cuts the earth connection.

You have broken the law! Under the Electrical Wiring Act, only a qualified electrician with knowledge of the IET Electrical Wiring Regulations is authorised to change a fitting! If you were such, you would not have asked the question!!!!

[Fat Tone]

You have broken the law! Under the Electrical Wiring Act, only a qualified electrician with knowledge of the IET Electrical Wiring Regulations is authorised to change a fitting! If you were such, you would not have asked the question!!!!

Thanks everyone. Brian, I will give you a call on your dodgy fruit to discuss our respective legal positions

[VJ]

Question is why is the neutral causing any current to flow to earth - guess faulty installation or condition of equipment somewhere in the house.

There are some devices that (claim to) need a connected earth to operate. I was considering a Sony LCOS projector, and it had clear markings in the manual that it MUST be earthed. As the apartment had no connected earth (not my current one, which has a connected earth since renovations), I opted for a model that did not have such a message.
Perhaps there are other devices with similar requirements, and the presence of one such a device explains it?

[Brian Ellis]

You do as I say, not as I do!

[Brian Ellis]

There are some devices that (claim to) need a connected earth to operate. I was considering a Sony LCOS projector, and it had clear markings in the manual that it MUST be earthed. As the apartment had no connected earth (not my current one, which has a connected earth since renovations), I opted for a model that did not have such a message.
Perhaps there are other devices with similar requirements, and the presence of one such a device explains it?

There are two possibilities:
1. The device has metallic parts that can be touched with an IEC Standard Finger, with ordinary insulation. In this case, an earth connection is required for safety. The device will work fine without one, but would become dangerous if there is an insulation fault. At your risk and peril! EU regs state that such devices must be wired with a local national plug with an earth connection. It is illegal to wire it with a 2-pin plug or without any plug.

2, The device has double insulation. This usually means that it is in a plastic case. If there are any metallic parts that can be touched with a standard finger, the part must have 2 distinct layers of insulation between any live part and the metal. It must be wired with either a 2-pin or a 3-pin plug, according to local regulations/custom.

The very bizarre and dangerous UK IET Wiring Regulations, which also apply here, are a special case as each appliance must have a plug equipped with a fuse in the phase wire. If the appliance is supplied with a two pin plug, an approved fused adaptor may be used. This is mainly because appliances drawing up to 13 A at 230 V (3 kW nominal) are plugged into a terrible 30 A ring mains and the flex leads would burn at this current, so the fuse becomes mandatory. Theoretically, the fuse should be dimensioned according to the current normally drawn and the cross-section of copper in the lead. In practice, as often as not, you find a 13 A fuse on a device which may use less than 1 A and with a very light flex lead. Of course, there is no protection as the flex will burn out before the fuse, causing a fire! As adapters inevitably have 13 A fuses, plugging a 2.5 W, drawing 0.01 A, 2 pin phone charger into such an adapter is hazardous. That is only one out of many hazards of these peculiar wiring regs.

[Tjalfe]

so, in the UK, you get 3 phases to residential?.. seems overkill, though good for running AC motors.

here in Canada, we have almost the same setup as in the states, though some stricter rules.
we get 240V single phase in, with a center tap, which is the neutral, hence we have 120V available for regular outlets, and 240V for larger loads, such as stoves, AC, dryers etc.
Here, the Neutral and earth ground are tied together in the main panel, and only the live wires are passed through the breakers. 15A@120V breakers for regular circuits, such as outlets and lights, so we have much less power available at the outlets than in Europe.
Once the breaker is off for a particular circuit here, there is no way to trip other breakers.. unless, as unfortunately is allowed by code, a circuit from a different breaker is in the same electrical box.. it can cause interesting sparks to fly, but never anything to trip the main breaker, which is usually 100A@240V, or in houses with electric heat, 200A@240V

[cjolley]

I think what they call what Brian talked about a "refection" here.
Oddly enough, you can sometimes detect a voltage between ground and neutral using a digital meter but not an analog meter because the tiny current the analog passes is enough to zero out the voltage.
I have a circuit like that in my house that a digital meter shows 30v on. But it won't light a test (low voltage) light between any combination of the leads if the switch to it is off.
A ground fault breaker will definitely not stay closed on a circuit like this.

[Umfriend]

I don;t think that the live-neutral-earth/ground set-up is a 3-phase system. 3-wire, yes. Pretty sure in Europe (and the UK) we get 1-phase current normally.

[cjolley]

That's the way it is in the US. But the article on Split Phase seems to indicate that Europe uses true 3-phase.
It's properly Split Phase, not 1 or 2 phase.

[Umfriend]

AFAIK, in the Netherlands, true 230V 3-phase is indeed used but it is not standard for households. Most get up to 1 x 45A @ 230V AC. If you need more, like I do, you can get 3 x 25A or even 3 x 35A @ 230V AC. But this is all split after the main breaker and the section breakers all run 3-wire single-phase AC.

Split fase is not used in Dutch households.

Farms, factories, larger shops and offices etc, I do not know about.