Why did my mains trip when I used a multimeter on a disabled socket?

Question

Here in the UK, I turned off a socket at the consumer unit. I confirmed the socket was dead with a socket tester. I confirmed the socket was dead via an appliance. I then unscrewed the socket and started testing all the live-neutral wires both ways with a multimeter set to 750 AC Voltage. Then, the consumer unit turned off electricity to the whole house and I was left thinking that I should never touch electrical circuits ever again.

But, for my curiosity, how did the consumer unit get to a state where it thought it should protect me from myself?

The box was double-socket and had two live, two neutral and two earth wires.

Answer 1

In the process of researching this, I found an excellent document (PDF) from Fluke, one of the big names in the tester business, that explains a lot of the safety issues. It even explains the "CE" self-certification mess (though admittedly to their own benefit, since their products have UL or similar testing). A good read, IMHO, for anyone new to electrical work.

I see two very different possibilities:

• Accidental Short

This is always a concern when messing around with possibly live wires. This is why, for example, some multitesters will include probe covers required for use with CAT III environments in order to minimize the exposed metal area and lower the chance of accidental shorts.

A multitester used properly (voltage) should, under normal circumstances, not cause an actual short, except for a "real" short - e.g., one metal probe hits both hot and neutral. But in the wrong mode strange things can happen.

• Ground Fault

A ground fault occurs when some current that should be going through the circuit (hot to neutral or hot to hot, depending on the type of circuit) instead goes to ground, possibly through a human being. RCD and GFCI are two different (but related) devices used in different areas to help mitigate this risk. You mentioned UK, which as I understand it typically has an RCD for an entire house (or possibly a few, but not one per circuit like the US).

While a ground fault typically refers to "problem in equipment or wiring", it can also happen with a transfer of current between circuits. That can easily happen in the US if GFCI protection is added to an existing house that has neutral wires connected/shared incorrectly - such problems need to get fixed in order for the GFCI to ever work. But such problems can also happen on a temporary basis if you cross wires in the process of testing. If you are on a whole-house RCD, mixing up the neutrals shouldn't matter, but a small leakage of a hot to ground via testing could trip the RCD.

Answer 2

The explanation that comes closest to fitting your narrative is an incorrectly configured multimeter causing a neutral to earth fault.

A common error with multimeters is to put the probes in the wrong sockets. On most multimeters the current shunts are permanently connected (via a fuse if you are lucky) to the corresponding sockets.

In the UK we commonly use single pole MCBs fed from a shared RCD*, turning off the MCB for a circuit disconnects the live conductor but leaves the neutral conductor connected.

Under normal conditions the voltage on the neutral relative to the earth is pretty small, too small to give you a shock, but the impedance is also pretty small so the current can be fairly large. Easilly enough to to trip a RCD.

So when you take an incorrectly connected multimeter and attempt to measure the voltage between neutral and earth. You create a short between neutral and earth and the RCD trips.

There are two takeaways from this.

1. If you want to work with just the MCB off without tripping the RCD you need to take steps to avoid accidental shorts between neutral and earth. Generally that means taping up the neutral as soon as you disconnect it.
2. You need to be very careful to ensure your multimeter is connected correctly before using it for mains work. Indeed I would go further and say you should never leave a multimeter with the probes plugged into the "current" sockets.

* How widely shared the RCD is varies depending on when the consumer unit was installed and the attitude of it's installer. There was a time when RCDs covering the whole installation were fairly common, then those fell out of favour in favour of "split load" installations where only some circuits were RCD protected (exactly which circuits varied, some put the RCD on everything except the lights, some only put it on the circuits for which RCD protection was required). Then the 17th edition came along requiring RCD protection for almost everything and the cheaper end of the installation market moved to dual-RCD boards with the more expensive end moving to RCBOs.

Answer 3

That can happen very easily by testing, or shorting, the neutral to earth (ground).

To the elcb for the supply that presents as an excessive trip current so it shuts off the suppy.

Best way to avoid that is to isolate the neutral for that circuit at the consumer unit as well, this avoids the neutral currents from other working devices being an issue.