RCD tripping

[Q]

Hi , Yesterday the house's electrics went haywire, causing the RCD at the consumer unit to trip. This is what happened:

1. Switched on washing machine and microwave, as well as usual lights - nothing unusual.
2. Returned from walking a couple of mile. Switched on kettle in kitchen (refurbished several years ago, with new sockets). Kettle immediately tripped the RCD, which I reset. I checked the socket - no dampness/loose wire, etc. Tried using the kettle in several other sockets around the house - all caused tripping. Tried microwave (brand new) - tripped in a number of sockets. Tried using a double-insulated hairdryer in the same sockets - no problem with tripping. No problem with using an old convection heater - worked in all sockets. Oven worked perfectly OK, as did fridge-freezer.
3. Today, kettle and microwave failed to cause any tripping - apparently back to normal.

Has anyone any idea what could have caused this? I thought initially it was limited to earthed appliances - but the convector heater is earthed, and worked OK.

Could I have built up a static charge while walking and discharged this at the 'faulty' socket when switching on an appliance?

In the past, we have occasionally had some tripping (latest was a couple of years ago, which resulted in some partial rewiring. Then, the tripping was limited to one appliance, and was certainly never seen with the kettle/microwace.

Any thoughts most welcome, especially suggestions to get around this if it happens again...

Stew

[tinnews]

Sometimes RCDs get 'sensitised' by repeated tripping (in my experience anyway). My guess would be that the kettle had temporarily got some water where it shouldn't have and it tripped the RCD, you tripped it a couple more times with the kettle and then other devices which are marginal from the RCD's point of vie tripped it as well.

A "double insulated" device which has no earth connection can't by itself trip an RCD with any sort of internal fault.

No, the level of current involved with 'static' on your body is trivial compared with the 30mA (or a little less) needed to trip an RCD.

[Q]

I need some advice please. Last Friday I used the washing machine and microwave perfectly normally, early morning. I left the machine running while I got on with some other jobs. An hour or so later, the machine had finished, I switched off, and plugged in my ageing kettle - it immediately tripped the consume unit RCD. I tried a couple more times in diferent sockets, with the same result. I then tried the microwave (brand new) - immediately tripped the RCD, again in a variety of sockets, in the kitchen, even upstairs! I then thought water might have got into a socket, so removed the face plates and used a hair dryer to make sure they were perfectly dry - I used the same socket that had previously caused tripping. I then plugged in the microwave to the same socket where I had plugged in the hair dryer, and it immediately tripped the RCD. At this stage I gave up! The next morning my wife used the microwave in the kitchen with no problem.

I read in the Telegraph that this frosty weather was exacerbating the incidents of drivers getting shocks from static build-up, and wondered whether these two scenarios are connected. Could I have generated static via my slippers, for example, and caused this to jump to earth when switchin gon the kettle or microwave, thus tripping the RCD? Today, as yet , everything's fine, but this problem has got me worried about the efficiency of the house's earthing system.

Anyone any comments/suggestions? I'd be really interested to hear what I can do to stop this happening again.

Thanks Stew

[Dennis Becker]

Could be a faulty RCD. Try swopping it over to see what happens.

Cheers

Dennis

[John McLean]

The RCD functions normally, by sensing that the line and neutral currents flowing through the unit are balanced, any leaking current flowing to earth resulting from deterioration of the insulation values downstream of the unit, will be detected and summated. (upstream faults can also cause an RCD to trip; but these are peculiar to the type of supply on the premises, I would ignore these at this time, since the arguments are complex).

Ideally an competent electrician with an RCD tester may determine if the RCD is OK, particularly the so called ramp test; but if the problem appears to have gone - albeit temporarily, this test won't be conclusive. An RCD although rated at 30mA on the box, can trip at much lower values.

The tripping mA's are additive for each leaky item - these are usually appliances with heating elements or have condensation/moisture around - freezers, refrigerators, washing machines, tumble driers, dishwashers, convection/microwave ovens, kettles and grills; but the notorious one is the iron, where fluff collects in the internals of the iron, around the heating element ends. Another test, which can be carried out, is specifically that for portable appliances - P.A.T.; but this also requires a special tester used by a competent electrician.

I don't know what circuits your RCD is protecting; the regulations basically require these for socket outlets which potentially be used outdoors, this could include a cooker but this is only required if there is socket outlet on the cooker unit - these have elements also!. Some electricians fit showers to these, as the manufacturers usually recommend this; but this is not required to the regulations.

If there is a recurrence, I would unplug all your appliances with an earth connection to them, check for moisture around the connections where possible; and plug them in one at a time. You may be fortunate and find an appliance problem; but it could also be that the RCD itself is faulty. Ideally of course, a competent electrician with the proper testers may be able to isolate the problem; but these faults can be difficult and time consuming to find.

Usually I find that the RCD is operating correctly due to a faulty item; (or items).

Regards.