Have my own ideas on this, but I wanted some independent thought.
Landlord living elsewhere reports 'no hot water, change immersion'.
When I get there I find a HW tank with an immersion, in a cupboard above a washing machine. No CH or boiler, only source of HW is the immersion.
Immersion is connected to a basic timer with an on/off switch & mechanical 'clock'.
Cables at the back of the cupboard are a right mess, looks like it was done by Stevie Wonder.
No power to timer.
Immersion & washing machine supply are each on a separate MCB (with nothing else).
Both MCB'S have tripped, takes 8 to 10 attempts to re set them. Once re set switching on the immersion timer or washing machine trips them again. If either MCB is re set the RCD trips immediately.
Immersion checked with DMM. Can't recall the exact resistance, but it wasn't open circuit.
Tenant reports that immersion was changed 10 months ago & 'failed' 8 months ago.
So two radial circuits - one feeding immersion, one feeding a socket for the WM?
When you move the toggle it immediately springs back? Any flash visible from the MCB or "poping" sound as it makes circuit? Any dimming of filament lights in the property as you close the MCB?
This only happens when the washer or immersion are also on?
Well, it needs a bit more investigation...
Assuming the MCBs are working (the pop/flash/dimming lights would be a good clue they are), then you must have something approaching a dead short to get an instant trip like that. If you were talking about a 16A MCB, then that's current in excess of 80A causing the trip. (100A for a
20A MCB). However the fact that you can get them to reset at all, would fly in the face of the above. So you could be looking at a dodgy pair of MCBs.
What rating are they, and what brand?
To fix methodically, I would disconnect both circuits at the CU, and disconnect the loads at the far end. Check the CU end to make sure there are no shorts between the L, N & E wire in the feed cable (check em for volts compared to neutral as well first to make sure they are not connected to another circuit). If that test is ok, then short the L & E at one end and measure resistance on a low ohms range - check the numbers correspond to a plausible length of circuit cable. Repeat for N & E and L & N. That should give you some confidence that the basic wiring is ok. An insulation resistance check if you have a megger would also be prudent.
With the circuit wires disconnected, and the CU "on" you can also see if the MCBs reset on their own. If they do, check the top connection actually goes live when they are mechanically switched on (that one can keep you guessing for a while - the MCB that only turns on electrically a subset of the times it turns on mechanically!)
If that lot is ok, then you can move attention to the appliance end. What does the WM do plugged into an extension lead on another circuit?
If you are going to do much electrical work on a commercial basis, I strongly suggest you buy a decent insulation resistance tester which has a low ohms range as well, together with earth loop impedance and an rcd tester. Then learn to use them and interpret the results correctly. You know it makes sense!
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