Naive electricity supply question ...

The socket is wrongly wired with the L and N swapped so that only the Neutral was being switched.

PS. Or perhaps the wires either entering or leaving the RCB are swapped so that even though the socket was switched off, the cable remained live.

Cutting through the flex could have created a N-E fault which might trip the main RCD on the consumer unit.

Correct. Ignore the other 2 earlier answers by Gareth, if it was a N-L reversal, then there would have been a bang/flash when you cut through the flex. It trips the RCD as the switch on the socket only turns off the Live supply to the appliance. When you cut it, you connected Neutral and Earth, there will be other things using power at the time, and for those few milliseconds until the RCD tripped, the return path for the neutral was through your pliers when they touched N+E. The RCD can detect this imbalance. Some newer sockets do switch both L + N.

I knew N & E touching tripped RCD but the thought as to why didn't ever cross my mind. I hadn't considered that being a common N it would still have current on it from other appliances but it's obvious now you've mentioned it.

Today I am a little wiser. :)

Cheers - Pete

I thought all MK sockets were double pole. Has this not always been the case?

Also, if you flipped the breaker at the consumer unit, that only interrupts the live. The neutral stays connected. Which explains why I had this happen when the socket was turned off and the whole circuit was de-energised at the CU.

(I suppose I should have measured N-E voltage before starting work, just in case of a fault where there was something on the neutral)

Theo

Well found, but why would anyone want to save 33 pence?

What is the story behind MK Base anyway? I assumed at first it was lower quality (I am always suspicious of the word 'value') and probably imported (I know they have a plant in Malaysia) but I see it offers a 20 year guarantee (same as Logic range). A better question might be to ask what is the point of the Logic range.

I have heard (from someone ex Scottish Power) that neon screwdrivers are disapproved of because if the bulb fails alive wire could be undetected. I don't know if that would constitute a false positive or a false negative. Is this correct?

Neon screwdrivers rely on a suitable current path to ground, which depends on many things - skin moisture, what kind of shoes you're wearing, etc. That's so much at risk of false negative there's useless for proving something dead. At best you can prove something live.

That's why electricians have proper proving units with their own power supply, and their own test sources that can be used to confirm correct operation of the proving unit before and after each test.

Ignore the answer above by Alan Lee because ut is likely today the N and E are the same circuit and only separated where the mains comes into the house.

Neither of those are plausible from the description.

Because you created a Neutral to Earth short, and neutral is a live wire[1], so the RCD can "see" a fault.

Also see my answer on the "Question for Electricians" thread of 21/05/23 for a detailed response.

I'd postulate that you have a 100mA type S double pole RCD serving as main isolator to the consumer unit with 30mA RCBOs serving the individual circuits in the house "Most" RCBOs only disconnect the live conductor. By cutting the flex you created a N to E short círcuit and sufficient imbalance fault current driven by Neutral potential would flow to trip the type S RCD. Double pole RCBOs do exist but mostly they take up a double width space in the consumer unit so seem to be akin to rocking horse s*it in the typical house installation prior to changes in BS7671 (I'm not up to date with latest changes )

In reality that makes some difference, but not enough to prevent a RCD trip...

It can be instructive to work through some examples:

Say you have a TN-S supply and your supply impedance at the consumer unit is 0.25 ohms, and you had a total load on all the house circuits of

That means (if we assume the L & N supply wires are of equal cross section), you would see half that drop in each leg of the supply. i.e. a

Now connect that 1.25V neutral to your earth, and a current will flow.

You could guestimate it - say your external earth impedance was 0.5 Ohm, and the neutral supply impedance in half the total supply impedance at

Even with TN-C-S, you can see the same effect. If your meter tails are

(I have used the "nominal" PME Ze figure of 0.35, in practice it is often quite a bit lower, which will make the problem more pronounced)

Well no, at best you can prove something dead.

A reading doesn't ensure it's live, at all.

This was very interesting though I thought John Ward was a bit clumsy. The Essentials range seems a bit crap, but not as crap as the Wylex plugs in another of John's videos that I went on to watch. MK Base (introduced in 2021) seems quite different. I wonder if the same comments would necessarily apply? Maybe MK have learned the lesson.