I'm looking at a CU at the inlaws and the live is running to the neutral bar and the neutral is running to the live bar which the mcbs are clipped onto. All wires to neutral bar are black. Does this mean the mcbs won't trip if needed ? I'm assuming I should switch the live/neutral feeds into the main CU breaker ASAP - live is now connect to the left side of the breaker - but any things to check first ?Thanks.

IF L and N really are reversed at the main breaker they do need to be connected correctly. This is not a DIY job for the amateur as it will have to be done live. Get your local sparks in to do the swap.

How are you verifying which incoming cable is live?

Better, since there is likely no earth leakage protection, why not shell on a Safe-T-Cut for them and get the installation chap to verify / correct the polarity. Two birds, one stone.

The MCBs WILL trip on overload even with the supply reversed but will leave all the wiring live and lethal.

If wired the wrong way, all your 'neutral' wires will be permanently live, essentially no fuse box connected and you can't turn them off.

I found this in my rented house when poking around with one of those 'live warning' screwdrivers.

Everything was live, all the time.

Thanks for the quick replies. I will have a local pull the live leads from the govt supply before I switch them over - I don't play with live cables!

I'm actually fitting a shower - so next question... The local spark left a cable from the CU To the bathroom for whenever we got round to fitting a shower. Its 2x2.5 so ok for a 4kw unit. But there,s no space in the CU for an RCBO. So I was thinking to run the cable off a 20A mcb from the main CU to a second small CU which has a 32A main breaker switch, 32A RCBO and a 20A MCB. Will add a 2m earth rod using 1.5mm cable from the shower unit (or maybe 2 runs of 1.5mm cable ?). Does this sound ok ?

Don't go over complex. If you're just going to add a shower, 500 Baht will buy you a 20A/25A 10mA (or 30mA) RCBO and a little box to put it in. Screw it to the wall near the existing CU.

Like this:-

If you're going to pull the cable at the meter ONLY pull the live (confirm it is the live) from the outgoing side, then there's no danger in getting them swapped when you put them back (which is likely how they got swapped in the first place).

Thanks. There are 2 CUs in house, one has the L and N in the correct place but the second on doesn't, so the switch over needs to be done at the incoming point of the 2nd CU.

Important you only mentioned two wires to shower - that shower must have a ground wire by law (and reasonable caution - I have had built in RCD fail on name brand shower).

Important you only mentioned two wires to shower - that shower must have a ground wire by law (and reasonable caution - I have had built in RCD fail on name brand shower).

Earth will run direct from shower to rod. No earth at the CU, unfortunately (but typical).