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Quick question about RCD plug adaptor

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Quick question about RCD plug adaptor

Postby Eddy Deegan » Sun Dec 01, 2019 7:05 pm

I have two power lines feeding the LCD floodlights in my garden and (on a separate cable) the outbuilding at the end of the garden. I only connect these when they are needed, and plug them into normal wall sockets through RCD adaptors.

The specific model of adaptors I'm currently using are these: https://uk.rs-online.com/web/p/products/7508441/

After working fine for a couple of years, one of the RCD adaptors (the one feeding the outbuilding) has started making a slightly strange noise when powered up. It still passes its own safety check but I don't like the noise. It sounds a little bit like whirring clockwork.

My initial reaction is to dispose of it and get another one, but I was wondering what that noise is, why its happening and whether it's safe to ignore it. I was going to open it up and have a look but it's put together with a couple of 'safety' screws for which I don't have a screwdriver bit.

The outbuilding is about 20m away from the house and although the armored cable has thick single-core conductors (there is a little junction box in the house which bridges that cable type to the conventional flex I attached the plug to) there is a small voltage drop, as I would expect, which is roughly negative 10v leading to somewhere in the range of 220-230v at the far end.

Max load is about 3Kw when the heater, lights and PA are turned on. There is a workstation down there that I use for private megablasts through a small JBL EON once in while, and my wife uses the PA for music as well.

Should I be using a different RCD model, replace the one that's fizzing/clockworking or is this a harmless noise that can be safely ignored?

The specs on the docs for it I've been able to find are pretty sparse:

Current rating of 13 A
• Voltage rating of 240 V ac
• Added protection against the risk of electric shock
• Non-latching - the power will need resetting each time it is used
• Automatically turns off once power to the appliance is terminated
• Double pole switching
• Reliable and continuous monitoring of power supplies
• 40 milliseconds of tripping time
• 30 mA tripping current
• Complies with BS7071:1992

Any advice appreciated, as I'm not really looking to bump myself (or anyone else) off at the moment.
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Re: Quick question about RCD plug adaptor

Postby Folderol » Sun Dec 01, 2019 7:12 pm

The buzzing might just be a relay buzzing (part of the switch on mechanism) or it might be it's contacts on the way out and arcing. Also look very carefully at its pins, and also the pins of the plug that goes in to it in case there is a weak spring there causing arcing. Of course there is also a possibility of the wires in the pug having worked loose.
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Re: Quick question about RCD plug adaptor

Postby Eddy Deegan » Sun Dec 01, 2019 7:19 pm

Folderol wrote:The buzzing might just be a relay buzzing (part of the switch on mechanism) or it might be it's contacts on the way out and arcing. Also look very carefully at its pins, and also the pins of the plug that goes in to it in case there is a weak spring there causing arcing. Of course there is also a possibility of the wires in the pug having worked loose.

Thanks Will,

Both plugs (the RCD and the plug on the end of the cable) are physically pristine, no sign of damage. In recent months there was a point where the internal cable had been given a tug by something (no idea what) and this broke the neutral conductor in the flex as well as dislodging the earth in the plug. I replaced the cable immediately of course.

I guess it's just on its way out... it does sound a bit 'arc'y, so I think best thing is to play it safe and get another one!
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Re: Quick question about RCD plug adaptor

Postby Wonks » Sun Dec 01, 2019 7:27 pm

Arcing would be my first thought as well, then a noisy relay.

The 'test' button is for earth leakage so will connect a resistor from live to ground, which should allow more than 30mA current to run direct ground, which should then get the protective mechanism to trip. It won't test if there's any issue with current flowing through the internal contacts.

I've been involved with hundreds of control panels (probably thousands if you count all the very small ones), and any noisy relay got thrown away. It certainly won't last as long as it should. So arcing terminals (definite fire risk) or noisy relays really means that it shouldn't be used any more.

But if you want to have a look inside, you can get sets of security bits dirt cheap e.g. https://www.amazon.co.uk/SILVERLINE-909 ... 20&sr=8-17

Always useful to own and have available.
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Re: Quick question about RCD plug adaptor

Postby Eddy Deegan » Sun Dec 01, 2019 7:53 pm

Good stuff... all makes sense. I've grabbed a set of the bits to satisfy my curiousity before I throw it away. Thanks Wonks :-)
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Re: Quick question about RCD plug adaptor

Postby DGL. » Sun Dec 01, 2019 8:28 pm

One thing I will add is from what I've heard pushing 3kW through a 13A plug for any length of time is pushing it a bit and will generally cause the plug to get quite warm. Personally I would switch to using an RCD spur, but in theory that is an electricians job (or so I believe to keep to current regs, though I could be wrong).
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Re: Quick question about RCD plug adaptor

Postby Eddy Deegan » Sun Dec 01, 2019 8:41 pm

DGL. wrote:One thing I will add is from what I've heard pushing 3kW through a 13A plug for any length of time is pushing it a bit and will generally cause the plug to get quite warm. Personally I would switch to using an RCD spur, but in theory that is an electricians job (or so I believe to keep to current regs, though I could be wrong).

Thanks DGL. The RCD does get a little warm when the heater is on fully but falls far short of anything I'd describe as hot to the touch. We are going to be having some electrical work done on the house at some point in the coming months, and a spur sounds like something worth discussing with the contractor. I've made a note to do so!
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Re: Quick question about RCD plug adaptor

Postby Folderol » Sun Dec 01, 2019 9:26 pm

That reminds me of a series of fires in the 1960s caused by immersion heaters in airing cupboards. These were always 3kW, which is 12.5A at 240V and were (of course) on for very long times. They were almost always connected via plug and socket. In the days of the old round pin plugs that wasn't really a problem as these were rated very conservatively at 15A. When the 13A style came out people just kept doing the same without thinking about it - including my dad.

Being at the peak of my electronic inquisitiveness I'd read about the problem, and told him about it, only to be told I didn't know what I was talking about. Having read of quick test, I pulled the plug out and touched the pins - not for long! Dad still wasn't interested when I told him. Eventually I tricked him by pretending curiosity at a 'thing' at the back of the airing cupboard, and when he leaned over to look I quickly pulled out the plug and held it on his arm. Needless to say he was not at all happy, but not a word was said. A few weeks later I saw the heater had been wired in. neither of us ever mentioned it.
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Re: Quick question about RCD plug adaptor

Postby wireman » Sun Dec 01, 2019 10:22 pm

There are a couple of other options which might be neater.

  • Replace the socket faceplate with a model with RCD built-in.
  • Use an in-line RCD on the link cable

If you get an electician they will likely make you take a curcuit back to the consumer unit, I can't remember if spurs are not allowed or are allowed and frowned upon nowadays.
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Re: Quick question about RCD plug adaptor

Postby Folderol » Sun Dec 01, 2019 10:56 pm

Any socket can have a spur, provided it isn't already a spur, and provided the total number of spurs doesn't exceed the sockets on the ring - that's been pretty much unchanged in the regs for years. At one time you were allowed two spurs. I would imagine turning a socket into a spur point would be regarded as even better.

However a contractor will probably try to persuade you to have a new line put in back to the distribution board - for obvious reasons.
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