Tripping RCD

[sparksmayfly]

I have recently erected a shed (yes i know a shed) in my back yard for use as a workshop so naturally i needed power.

Having put in many sockets and switches in the past this seemed like an easy thing to do.

I ran cable from my house down the garden, this was under the ground in an insulated conduit, and into the shed.

From here it goes into a 45 amp switch. Out of there i have one cable for the sockets and one for the light.

All connections being good i switched the power back on at the consumer unit in the house. All good so far.

I had been using the light and the sockets for a few days without problem.

After about 4 days, when turning OFF the switch in the shed i discovered the rcd in the CU in the house had tripped.

I reset it and went back to check the shed, all worked ( as you would expect)

Again a few days went by without problem. I turned the power Off in the shed one night and again it had tripped!

It only happens when i turn the power OFF in the shed. and not all the time.

Can anyone throw some light on this? I can seem to find what would be causing this ( having checked all the wiring many many times!!

 

[Steve]

Not that this has anything to do with your problems, but what size breaker is at the house? What size cable is the run to the shed in? and what type? What is this "45amp switch"? like a cooker switch? Are the lights on any sort of fuse or MCB at the shed? What sort of appliances do you run in the shed? Are they on when you turn the main switch off?

And there's nothing wrong with sheds (the wooden variety). I'm sure if you ask him, ban will explain his name fully to you!

 

[Nickthedentist]

Hi,

I've had this kind of thing happen when the isolator disconnects neutral fractionally before live.

I'd double check that it's wired the right way round, i.e. Lin, Lout, Nin and Nout connected as embossed on the isolator.

Nick

 

[Adam_151]

I've had this kind of thing happen when the isolator disconnects neutral fractionally before live.

I've heard of this before, but I can't understand how this would be possible, unless the equipment downstream of the switch for some reason leaked quite a bit to earth if it had phase, but no neutral, but I can't see why this would be, or how such equipement would be able to get approved

Maybe the answer has got something to do with reactance , too much to be thinking about on a cold sunday afternoon though >.<

 

[sparksmayfly]

Hi crafty, the breaker in the CU is a 30mA rcd and the switch in the shed is like a cooker switch (unfused)

Appliances are drills, jig saws, sander etc . There are never any appliances on when i switch off the power.

There is only one flouresent strip light on at the time the power goes off.

cable is 10mm household flex, (earth, neutral and live)

There is no sort of breaker in the shed.

I understand having a faulty circuit would cause the rcd to trip when power is turned on, but i have never experienced it when you turn the power off.

I have created circuits like this many times and never had a problem?

 

[mapj1]

The problem is, as alluded early neutral break - and here is the mechanism.
If the neutral is interrupted, and the live is not, both live and neutral cables rise to ~230V, now the leakage current to earth has been doubled, as instead of L-E capacitance with 230V accross it and N-E capacitance with <10V accross it, we now have twice as much wiring that is live.
If the system was halfway to tripping (say approaching 15mA leakage for a 30mA trip...) it will pop out at that point.
The fact this condition persists for an instant is no help, as now a high frequency transient is also created, and the capacitance passes higher frequency currents with greater ease than 50 Hz, so the trip sees much more leakage than the steady state. At that point it cuts the power. The real solution is to have a separate RCD at the shed side of things, and supply it from a non-RCD feed.

 

[sparksmayfly]

Thanks mapj1.

I was already thinking that this may be the answer.

My intention was to install a CU in the shed thus preventing ther house RCD from tripping.

Thanks for your help and advice.

 

[Lectrician]

You still hav not stated what the cable size to the shed is, and what the actual overcurrent rating is in the house?? A 30mA RCD doesn't provide overcurrent protection on it's own.

Also, are the lights fused down within the shed??

You maywell be able to sort the tripping problem, but still be left with an unsafe installation.