I have an RCD socket outlet feeding my workbench - where I do electronics type stuff. One of these:-
Would a socket with a lower fault current limit sort this - and who makes one? Not much point in having this one if the main one is going to trip first. Although it does pre-date the split load CU.
Dave,
Don't automatically blame your local bench RCD. It is possible that there is enough leakage in the rest of your installation to be near the threshold of your split CU RCD and a few mA extra from your ATX PSU tripped that whilst being well below the threshold of your local one.
Maybe put a controlled leak (suitably rated variable resistor) on the circuit before your bench and see how much current it will take before the CU trips out?
The main problem I find with RCDs and experimental bench is the effect of mains filters in test equipment can cause nuisance tripping. I'd rather work carefully via an isolating transformer.
Bob
If you generated, say, a 1A leak, then it's a toss-up which one trips (although for any given pair, it will often be the same one). The tripping currents of the devices (providing less than the 1A leak) won't make any difference. (Can even get both tripping sometimes.)
To get discrimination between RCD's, you have to use time-delay on the up-stream one. If tripping everything is going to be a problem, I would think about running a non-RCD protected supply to the RCD socket.
Yes - although I don't get nuisance tripping from the main one. Oh - it wasn't a few extra mA from the ATX - try an earth to neutral short via the case due to a trapped cable. ;-(.
Worth a try.
I do have an isolated output. But wasn't using it in this case.
That's the best option, but there is an easier one if you're already using an iso transformer. Simply feed your rcd socket from the iso transformer. Now any earth leakage after the rcd socket isnt even seen by the CU RCD.
NT
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The additional leak doesn't have to be big. The RCD in the CU has the leakage of *all* the circuits connected through it to contend with. The one on your bench, down stream of the CU one, just the bench circuits. If the house is sitting at say 20mA and the bench 5mA, 15mA of extra leakage at the bench will trip most likely trip the CU RCD (20+15 = 35mA) not the one on your bench (5+15=20mA)(*).
To give you the discrimnation the CU RCD needs to be a time delayed one or feed the bench RCD on a radial taken before the CU RCD. I'd go for the latter.
(*)Assuming a 30mA RCDs and they trip precisely at 30mA.
That's the best option, but there is an easier one if you're already using an iso transformer. Simply feed your rcd socket from the iso transformer. Now any earth leakage after the rcd socket isnt even seen by the CU RCD.
I fail to see what advantage there is fitting an RCD after an isolating transformer, as by definition the secondary is isolated from earth, and you can earth ether side with impunity. A boon when connecting test-equipment to live chassis radios and TVs under restoration.
it prevents the upstream rcd tripping. You can also have your supply isolated or neutral earthed as you please, and the socket rcd will still work in the latter case.
NT
In which case it is a "race hazard" - either or both could trip. This is why TT installs with cascaded RCDs use a time delay type at the head end to ensure discrimination.
Added to uk.d-i-y.
As your CU is less that 12 months old I would expect it to have two RCDs. Is it the same RCD that trips with both the pressure washer and the waste disposal unit?
Yes - ground floor ring main.
OK. Is it possible to try either of the two appliances out via a circuit from the other RCD to see what happens? eg extension lead from upstairs. Note that the circuit MUST be fed from a different RCD than the one that feeds the ground floor ring not just a different circuit.
I strongly suspect that you have a live-neutral or live-earth low resistance on one of the circuits protected by the RCD that covers the ground floor sockets.
There is no reason why either of the appliances should trip an RCD unless there is a fault somewhere.
I strongly suspect that you have a live-neutral or live-earth low resistance on one of the circuits protected by the RCD that covers the ground floor sockets.
There is no reason why either of the appliances should trip an RCD unless there is a fault somewhere.
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RCD's can appear to be over-sensitive. I have a garden fountain pump that trips the RCD when it is turned off! Changed the pump - still the same. Has worked fine for years, but the RCD needs to be reset before it can be used again.
Chris R
The thing that your waste disposal and pressure washer possibly have in common is they typically have induction motors. These have quite high inrush current when first turned on (can be anything from 5 to 9 times their operational current). If your RCD is operating close to its trip current anyway, this inrush can be enough to push it over the edge.
See the sensitising RCD section here:
The fault may be not directly related to the appliance that apparently causes the trip.
Thanks for the advice and the link John. I'm not sure that I have the necessary expertise to sort this out for myself. The electrician who installed my CU lives on the same estate as me - just around the corner, so if I can't resolve it myself I may ask him to come and take a look.
The 17th edition of the wiring regs requires most domestic circuits to be protected by a 30mA RCD.
There is no reason why a kitchen sink should be "earth bonded" and any waste disposal unit that requires an earth will get it's earth from the electrical suppy to the unit and not from the sink.
You may be able to do some preliminary stuff to help him track it down. Stuff like unplugging all the other appliances from the circuits that are protected by the RCD that trips, and then seeing if you can still get it to trip with just the pressure washer or waste disposal. If you can, then that would point to a neutral to earth short somewhere in the wiring.
Okey doke -I'll give that a try. The waste disposal trip is infrequent and so I don't think I can do much with that - but the pressure washer trips it every time I first switch it on. Can't quite understand why it should do it the first time I turn the washer on - but not the second time after resetting the RCD.
Tomorrow I'm going to cut a one meter extension lead in half and insert a 10ma RCD in it, so I can use it on as a sort of RCD leak current tester, and use it when using tools etc on extension leads, so it will trip rather than the main 30ma RCD in the consumer unit.
You could use it on your 2 bits of equipment and see if they trip it.
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