View Full Version : RCD tripping

28-12-2016, 09:51 AM
Every now and then, when I turn off the main isolation switch on the Boxford HSR, it takes out one of the RCDs in our consumer unit. It's not every time, so I don't believe it is a fault, but it's annoying, especially as it takes out half the household electrics. It never seems to do it on power up, just power down.

There is just about enough space in the main consumer unit to give the workshop its own RCD, but would a surge suppression gadget on the mains supply to the HSR be enough to cure the problem ?

The Boxford lathe is on the same ring but has never caused a trip when powered up or down.

There is a small consumer unit in the workshop, but I understand you can't daisy chain RCDs, so I can't fit a local one to limit the blackout.


Web Goblin
28-12-2016, 10:20 PM
You could fit another rcd in the workshop and I would recommend that you have one. The best idea may be to get the installation for your workshop changed to a TT installation and fitted with rcd protection. This should prevent any faults in the workshop from tripping the protection in the D.B.
Do you a split consumer unit in the house? Does it have one side protected by and rcd and the other side not? If so then supply the workshop from the non rcd side via an mcb then fit an rcd at the workshop D.B.
There are a few options for this and if your aren't competent in wiring then get someone who is to look into it. If the wiring is already in place and suitable for the job then its not a big change.

01-01-2017, 10:12 AM
RCD's are not about surges but leaks to earth. I'm not expert on building wiring. Could it be something like a dodgy suppressor filter that generates some sort of extra pulse to earth that trips the switch.

Sent from my phone so mind the autocorrect.

01-01-2017, 12:06 PM
I understand you can't daisy chain RCDs, so I can't fit a local one to limit the blackout.


RCD is an earth leakage protector and you can put one local unit, but you must make sure it is tripping faster (= specified for less leakage current) than the main one. There are even sockets which protect just one socket, so of course there is local protection possibility, but regardless how you do you should (or at least I would) find out why it is tripper, not just accept the fact that it trips now and then. There must be something wrong somewhere in your electrical installation, or some wires or switches get wet during use, or something similar. Perhaps the motor, perhaps something else, but definitely something is wrong if it trips.

01-01-2017, 12:40 PM
Typically a master RCD is fitted rated at 100mA. This won't save your life but is an overall protection of gross faults. After that any amount of 30mA RCD's are put on separate circuits. I don't know if you can say get a 60mA RCD. Or maybe you need to fix the equipment. The 30mA will cut out at less but with a longer fault. It's about how much for how long that the human body can withstand.

Sent from my phone so mind the autocorrect.

01-01-2017, 02:02 PM
10, 30 and 100mA seem to be the common RCBO's. You could put a 10mA on the kit, but you would probably get it blowing all the time. If you have carbon build up in the motor you can get a trip from the back-emf leaking to earth, so it wouldn't hurt to blow out the motor, assuming it is the brushed type. Else you might have damp dust in the electrics somewhere, so again a clean out with compressed air could help.

Happy New Year


21-01-2017, 11:52 PM
Many thanks for all the comments.

Today the RCD blew mid-morning, without me even having been down to the workshop. By a process of elimination, I have found that an external 13A socket we never use had suffered from water damage.
I think that the water has entered the MICC which provides the supply to the workshop, and now the trip will not stay energised, even with the workshop consumer unit disconnected.

Looks like I need to either get the MICC repaired or have the whole lot replaced with SWA :-(

It may be that this water seepage has been going on for a while, so may have contributed to the earlier RCD trips.