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I just put some lamps on and got the fault-current circuit breaker triggered. While skinning the earth conductor the knife touched the neutral.

I was a bit puzzled: I thought that happens only under the following conditions:

  1. Live goes to ground and
  2. Live is actually live (the light switch was turned off).

Apparently those assumptions were wrong. So my questions:

  1. What kind of fault current can there be from neutral to ground?
  2. If neutral has tension relative to ground, just out of curiosity, how much tension is it usually?

I'm in Germany in case this matters.

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    \$\begingroup\$ "I'm in Germany in case this matters." - actually it does: the EU, like here in Australia and England chose to deploy lethal 220V / 240V in the home. So be careful! Not one of Germany's brightest decisions I must say: 115V as in Japan or USA makes heaps of difference, seldom resulting in a fatality. In contrast, 240V couldn't have been better designed to kill someone: at significantly higher voltages (300V and over), the heart tends to stop dead without fibrillation, so if someone isn't burnt by a shock, it is relatively easy to resuscitate them with CPR ... \$\endgroup\$
    – WetSavannaAnimal aka Rod Vance
    Feb 4, 2014 at 7:49
  • \$\begingroup\$ In contrast fibrillation means that CPR will need to be applied to a 240V ahock victim without let-up until a defibrillator can be gotten to: it's almost impossible to resuscitate a 240V shock victim by CPR alone. \$\endgroup\$
    – WetSavannaAnimal aka Rod Vance
    Feb 4, 2014 at 7:50
  • \$\begingroup\$ @Qmechanic Thanks, I wasn't aware of that. The question should probably be moved there. \$\endgroup\$
    – John
    Feb 4, 2014 at 10:49
  • \$\begingroup\$ Test for live: Use the best tester you have available to be CERTAIN that all circuits are "dead". | THEN ground them :-). Wearing safety goggles advised. Turning head away a minimum good idea. If you are SURE the circuit is dead then this can do no harm. Right? When the circuit is live despite very test this will save your life. \$\endgroup\$
    – Russell McMahon ♦
    Feb 4, 2014 at 14:30

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As you are in Europe, the breaker is most likely an RCD ("Residual Current Device") triggered by approx 20ma imbalance between Live and Neutral.

As the Neutral wire has some impedance, it will show some small voltage due to currents returned from other circuits in the house back to the substation, even though the circuit you were working on was disconnected (It was, wasn't it? :-)

So a momentary short from neutral to earth would divert some neutral current to earth, introducing the imbalance which tripped the breaker.

It seems unlikely that you actually shorted a phase to earth, as you did check for "live" voltages. However as the neutral current could be quite large and the earth wire has a low impedance, that imbalance could have been several amps despite the low voltages involved. So it is worth taking the whole house supply off before doing this sort of work.

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  • \$\begingroup\$ Of course! The impedance of the cable will make it have some voltage (as the circuit wasn't disconnected, only the light switch turned off - and by that I can now conclude that the light switch can't have been bipolar as asked by jlandercy). I could really have thought of that myself. Thanks for your answer. \$\endgroup\$
    – John
    Feb 4, 2014 at 11:59
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It seems you are doing dangerous things at home. What a knife (is that the only tool you have?) is doing close to wire without shutting off power (I mean be sure at 100% that there is no active phase).

I strongly advise you to stop this kind of activities unless you exactly know what you are doing, use proper tools and prepare your work as professional does.

Anyway tempting to answer your questions:

1) Is your light switch bipolar? Does it break two wires or just one? If it is unipolar, there is still an active wire when you are working. You must be aware of that before working on the circuit. You are not supposed to discover it after.

2) What do you mean by neutral? A blue wire? What is your earthing system? If you are in a domestic installation, you likely have a TT schema and therefore you have no neutral distribution but two active phases.

3) A fault-current breaker will switch off when 30mA or 300mA (depends of its characteristics) flows from any phase to earth. If there is no load, it is just limited by the wire impedance, make your own conclusion. In your case, I am pretty sure you simply shortcut a phase to ground and the circuit breaker saved your life (I am not joking, domestic injuries - including death - with electricity happens the way you did, not like in your last post).

4) It is a common mistake to believe that neutral is not an active wire. For a three phases load no current will flow through neutral if all phases are balanced (this never occurs in reality), this case is a theoretical exception. Other way, such in TN earthing system, neutral is collecting a huge amount of current. Neutral IS an ACTIVE wire.

When dealing with electricity you must at least use protection (there are circuit breaker, you haven't used them), correct tools (where is your multimeter to check that the circuit was broken?) and plan your actions before performing the maintenance.

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  • \$\begingroup\$ I did check that no conductor was live with one of those screwdriver-testers (don't know what their proper name is in either German or English). I can't really say much about the electrical installation where I was doing it, it wasn't at my home. Thanks for your answer, I've got some reading to do. \$\endgroup\$
    – John
    Feb 4, 2014 at 10:53
  • \$\begingroup\$ This kind of detector reacts to variation of electric field and use your body in derivation to ground. Information it gives you is binary and might be not accurate (false negative are common). Most important, this kind of device cannot be used to confirm absence of potential. To check that you need a Voltmeter. By the way Ground wire must be handled with the same care that any phase or neutral, because you cannot guaranties that your circuit is sane, specially in cities. \$\endgroup\$
    – jlandercy
    Feb 4, 2014 at 13:57

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