RCD trips but not the mcb

[sups]

Hi
I moved into my home 2 years ago, but before I moved in I had the place rewired top to bottom. The electrician installed a dual RCD consumer unit, and everything seemed to work well. A few months back my kettle developed a fault and it kept tripping the RCD, I was a little taken back because I would have thought that the MCB for the kitchen should have tripped first. I bought a new kettel and all was well again.
But last week I had a separate guy come and install a socket in the garden. The way he did it was to take a spur from a near by kitchen socket, but he has also installed an isolation switch. The garden socket has a built in RCD, but here something strange is happening. The socket has a test button, and each time I press the test button the RCD serving the kitchen trips.
When pressing the test button I would expect the outside socket to cut off and not effect the circuits inside, and if it did then the kitchen MCB should trip before the RCD....right?

When I was having the kettel problem I read something online about an earthing issue possibly causing the RCD tripping instead of the MCB, but I can't see why pressing the test button on the outside socket is tripping the RCD too.

Can someone tell me if this is normal?

 

[AndyPRK]

Yes it's normal.

Rcd s trip when there is a leakage to ground.
Mcb s trip on over current

 

[endecotp]

A few months back my kettle developed a fault and it kept tripping the RCD, I was a little taken back because I would have thought that the MCB for the kitchen should have tripped first.

Why would you think that?

But last week I had a separate guy come and install a socket in the garden. The way he did it was to take a spur from a near by kitchen socket, but he has also installed an isolation switch. The garden socket has a built in RCD, but here something strange is happening. The socket has a test button, and each time I press the test button the RCD serving the kitchen trips.
When pressing the test button I would expect the outside socket to cut off and not effect the circuits inside, and if it did then the kitchen MCB should trip before the RCD....right?

No, wrong.

What's happening is as expected.

 

[333rocky333]

Its the way the test mechanism works on rcd sockets, it seems to inject current to earth thus also tripping the house rcd, I recently took 3 sockets out as every night they used the test button to turn the slicers off and hence tripping the supply, causing loss of the whole deli circuit,

 

[winston1]

The guy who installed the garden socket should not have put one in with a built in RCD as there is already one serving it.

 

[333rocky333]

Regarding the kettle most fail by the element shorting to earth, the current needed is minimal to trip the rcd, whereas to trip the mcb, current would need to be a lot more, so the rcd in most cases would trip well in advance of any mcb.
A live to neutral fault would likely trip the mcb but these faults are not as common as live to earth faults in my opinion

Did the installer actually test the new garden socket, surely he must have noticed it tripped the kitchen, or did he not care

 

[Jackrae]

From previous posts you'll also find that the test button doesn't necessarily produce a fault to earth. It may be configured to produce an out-of-balance current through the RCD senses coil.

 

[sups]

Thanks all, I feel much better.
So for my simple brain...RCD trips when there is an earth to neutral fault.
MCB trips when the draw of current on a circuit is higher than what's rated on the MCB.
Is that accurate?

333rock333
I wasn't around when the sparky did the install. I can only assume he did the relevant tests

Winston1
The RCD socket was used just in case I decide to install another socket outside. The plan would be to dasiy chain it off of the current RCD socket.

That brings up another question. Is this dasiy chain approach the right thing to do if ever wanted another socket?

 

[bernardgreen]

So for my simple brain...RCD trips when there is an earth to neutral fault.

It should trip on any leak to earth. The situation that trips it is that when the current on the Live through the RCD is more than 30 milli Amps different from the current on the Neutral through the RCD

It might not trip on a Neutral to Earth fault if there is no electricity being used in the house. Then when an appliance is switched on the RCD trips. One assumes then that the fault is on the circuit supplying that appliance. It might be a Neutral to Earth fault on that circuit but it could also be a Neutral to Earth fault on another circuit fed by that RCD. The MCB on that faulty circuit could be turned OFF but the RCD would still trip.

 

[Taylortwocities]

The RCD socket was used just in case I decide to install another socket outside. The plan would be to dasiy chain it off of the current RCD socket.

That brings up another question. Is this dasiy chain approach the right thing to do if ever wanted another socket?

Does the "isolation switch" have a fuse? If so, you can add additional sockets - but note the max current will be limited by the fuse.
If the isolation switch has no fuse, then you cannot add more sockets.

 

[winston1]

Winston1
The RCD socket was used just in case I decide to install another socket outside. The plan would be to dasiy chain it off of the current RCD socket.

That brings up another question. Is this dasiy chain approach the right thing to do if ever wanted another socket?

But even if you did that (which you can't if the first socket is spurred off a ring) you still don't need an RCD socket as the circuit is already protected by the RCD in the house. Having 2 or more RCDs protecting one circuit is bad practice as you don't know which will trip first or maybe both/all together under fault/test conditions.

 

[JohnW2]

From previous posts you'll also find that the test button doesn't necessarily produce a fault to earth. It may be configured to produce an out-of-balance current through the RCD senses coil.

The test button of 'standard' RCDs, such as one finds in CUs or uses as 'standalone' devices does not create a fault to earth (well, I've never come across one that does). However, RCD sockets and plug-in RCD adaptors (and maybe also RCD FCUs) often/usually do create a fault to earth when the test button is pressed.

I don't know why that is. I suppose it might just be that (particularly in the case of plug-in RCDs) it has been done so that the user is alerted if a satisfactory (or any) earth is not present.

Kind Regards, John

 

[sups]

Thanks... So far things are Woking as they should then... Even though its not as I expected.

This might be a basic question but anyone able to explain why I couldnt hang another socked off of the RCD socket? Is it a regulation thing?
The isolation switch does have a 5A fuse.

 

[Taylortwocities]

This might be a basic question but anyone able to explain why I couldnt hang another socked off of the RCD socket? Is it a regulation thing?
The isolation switch does have a 5A fuse.

5AMP? That's enough for lighting, but too low for a socket. I would have expected a 13A fuse. You might like to ask your electrician why he has only installed a 5A?

I was asking if there was a fuse, because your electrician has spurred of what (I am assuming) is a ring final circuit.
You can only have one socket spurred from a ring final. If you added another socket, that would be a spur off the spur, and that's not allowed. It's to stop the cable being overloaded.
However, as you have a fuse in the isolation switch, you could add as many sockets as you like as the max current would be the size of the fuse.

 

[ban-all-sheds]

This "separate guy" is sounding less and less like a real electrician with each bit of new information.

He installed an RCD socket when there was no need.

He clearly didn't do proper testing, or if he did, as Rocky said, he didn't care.

He thinks that an RCD socket provides RCD protection to sockets spurred from it, which (a) it doesn't and (b) would in any event no more be needed than it was for the first socket.

He's put a socket on a 5A fuse.​

Sups - has this clown provided you with a certificate of BS 7671 compliance? Have you paid him yet?