Don’t demonise RCD’s

When we moved to WA we were told quite early about regulations regarding RCD’s (or Residual Current Device’s). While this seemed an imposition we found that both the house we rented and the one we purchased had been fitted.  (a striking difference to the regulations we were familiar with over east.

I should state upfront that I am not an electrician. I have some related qualifications so I know that (unlike plumbing) electricity miraculously flows uphill and downhill. I also have the sneaking suspicion that our house has had an amateur let loose on it in the past and I need to know if the house is OK.

So… the story goes like this.

My mother came to visit, she waxed lyrical on slow cookers and was compelled to buy us one. Three delicious meals later, and after the cooker had a prolonged rest (during summer) I dusted it off and switched it on preparing a Mexican dish. Music blaring loud I popped the meat and sauce into the cooker and grooved around the kitchen to get the “special herbs and spices”.

10 minutes later… Silence…

No music, no fridge, no anything! Certainly no little red or green light on the slow cooker. Hmm, obviously a glitch, I’ll just go reset the Circuit Breaker. Nope not the Circuit Breaker, hmmm the RCD switch should be up shouldn’t it?

Six trips back to the RCD later I finally resorted to the old cast iron pot and had relegated the Slow Cooker to the cupboard. And now the story gets strange.

First suspicion, the cooker. Tried it again later and found it just as bad. Plan B was to ask our friendly neighbours if i could try it in their house. They cooperated and 4 hours later we had a great Mexican dish, shredded beef burrito’s with melted cheese and iceberg lettuce with honey lemon and sesame seed dressing (but this is not an episode of MKR). Now that I know it works in another house with an RCD the problem is still not solved. Buoyed by the Burritos I resolved to try it again.

Second suspicion was the RCD. OK So I have a pool that runs on a separate circuit and its own RCD. Six trips back to the pool filter later it was looking like the cooker again; cast iron pot.

OK so a year later I drag it out on a slow weekend for a post mortem. The problem does NOT occur when I first turn it on, so I began to think it must be device capacitance (for a great article on how this can be the bugbear of appliances and RCD’s see this page: http://www.marcspages.co.uk/pq/3342.htm). I needed 100 Ohms connected from earth to chassis to get it working; nope not good enough –  resistance has to be less than 1 Ohm.

Image of element connection points
Before and after, showing arc point of short circuit. The structural band (with holes for power entry) is held together by a bent over aluminium band (visible at top and bottom) both are directly connected to the chassis. The wire entering into the bakelite sandwich is live, small wonder it arced where it did, and not at the wire entry

Dig deeper. It turns out that the “element” is a band of aluminium, bakelite and an “insulated” filament. The pictures now speak for themselves. I pinched back the aluminium band, rotated the bakelite and filament section to line up with the holes and tested again. No more trips to the switch box. The appliance is definitely faulty and validates the use of the RCD. This could have made the chassis go live and only didn’t because the arc required a higher voltage which it did during the inductive load during switch off/on.Now I need to tell the neighbour to test his RCD.

2
The other side of the element. Again the wire exit is perilously close to the chassis earth point

I just hope this gets out to all those people out there that seem to blame the RCD for their woes. There seems to be a general push to retrograde to an old fuse or worse. Keep the RCD.

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