Is a 15mA socket rcd available?

[dave_104]

Hi,
I wondered if something like a 15mA SRCD is available? I have a socket near my front door which would be useful for external things like jetwashers but I want to avoid tripping my house ring (which is on a 30mA RCD) and to help give discrimination on the circuit. I appreciate that the main RCD may not trip at exactly 30mA of course.

thanks, Dave

 

[sarkyspark]

there are 10mA ones availabl.

 

[dave_104]

Thanks Sarky I just found one - Greenbrook K22WPA10-C. Bit pricey but obviously not as commonplace like the regular Screwfix 30mA cheapies, but obviously it’ll solve the issue.
Cheers!

 

[EFLImpudence]

I have a socket near my front door which would be useful for external things like jetwashers but I want to avoid tripping my house ring (which is on a 30mA RCD) and to help give discrimination on the circuit.

It doesn't work like that.

I appreciate that the main RCD may not trip at exactly 30mA of course.

Actually it is required to trip anywhere from 15mA up to 30mA - usually around 27mA.

How do you plan to restrict any faults to 15mA?

 

[flameport]

but obviously it’ll solve the issue.

It will solve nothing and any money spent on it will be 100% wasted..

 

[ericmark]

The 10 mA RCD sockets we had at work would trip the 100 mA RCD's every time the test button was pressed, so a bit useless, these were MK so not cheap.

 

[plugwash]

As has been said 10mA RCD sockets are available. Standard RCD sizes seem to go up by roughly a factor of 3 each time, so you get 10mA, 30mA, 100mA and 300mA in regular fixed trip current RCDs.

But a lower trip current is only helpful if the problem comes on gradually. If the problem appears rapidly then it is very likely that both RCDs will trip at the same time.

There exist "time delay" RCDs which are used in large TT installations and some models even have adjustable trip currents but I'm pretty sure they don't meet the requirements for "additional protection", so they aren't much use in this scenario.

Realistically if you want a socket that doesn't trip out other parts of your installation then it likely needs to be fed from it's own dedicated RCBO in the consumer unit.

 

[bernardgreen]

The 10 mA RCD sockets we had at work would trip the 100 mA RCD's every time the test button was pressed,

Did you find out the reason why the 100 mA RCDs tripped ?

The usual test button circuit simulates an Earth fault in the RCD but does not create an Earth fault.

The RCD being tested does not have a Earth so it is impossible to create an Earth fault for the up-stream RCD simply by pressing the test button.

 

[ericmark]

I have the old MK socket so can test, I assume a test button also checks and earth is present so Button will test to earth not just an imbalance?

 

[JohnW2]

I have the old MK socket so can test, I assume a test button also checks and earth is present so Button will test to earth not just an imbalance?

Yes, we've been through this before. Many/most RCD sockets (and I think also RCD adapters) will not even reset without an 'adequate earth connection' - which I suppose makes some sense.

Also, as said, unlike standard RCDs and RCBOss, the test button in these things seems to create a 'fault to earth', and therefore are quite likely to trip any other residual current devices with which they are in series.

Kind Regards, John

 

[EFLImpudence]

Many/most RCD sockets (and I think also RCD adapters) will not even reset without an 'adequate earth connection' - which I suppose makes some sense.

It does, but how does it know?

If it uses an earth connection (CPC) for the test button mechanism yet without that earth connection it will not switch on (reset).

Does that mean there must be a small current on the CPC all the time?

 

[JohnW2]

It does, but how does it know?

We've discussed that 'how' question in the past, but without really coming up with any convincing answers

If it uses an earth connection (CPC) for the test button mechanism yet without that earth connection it will not switch on (reset). Does that mean there must be a small current on the CPC all the time?

One might suspect so. As you imply, unless it's constantly trying to put current through the CPC, I can't see how it could know whether an 'adequate earth connection' was present.

However, I can't remember details of the experiments I undertook a long time ago. I certainly could not reset one of these devices without an earth present, but I can't remember whether or not it would 'remain reset' if one removed the earth connection after it had been reset. In other words, it could be that the earth connection (hence sending current down the CPC) is only required during the resetting process, and not after it has been successfully re-set. If/when I have some time, and can find suitable devices, I'll look into this.

Kind Regards, John

 

[flameport]

It does, but how does it know?

I haven't looked in any, but the most likely is just voltage sensing between L&E - if it's ~230V then it will switch on, if not then it doesn't. Wouldn't need any current to flow, just voltage detection which would be a high resistance (Mohms) potential divider on L-E connected to a single input pin on a cheapo microcontroller or equivalent custom chip which will be in it anyway.

 

[JohnW2]

I haven't looked in any, but the most likely is just voltage sensing between L&E - if it's ~230V then it will switch on, if not then it doesn't. Wouldn't need any current to flow, just voltage detection which would be a high resistance (Mohms) potential divider on L-E connected to a single input pin on a cheapo microcontroller or equivalent custom chip which will be in it anyway.

That's certainly a possibility.

However, I seem to recall (but may be wrong - it was quite a long time ago) that in order to allow re-setting, there had to be a pretty low impednce to earth from the CPC (i.e 'an adequate earth'). Thst would agin make some sense but, if true, I can't see any way it could work without sending a significant current down the CPC.

However, as I recently said, what I can't remember is whether, once reset with 'an adequate earth' present, it will trip if one removes the earth connection.

Kind Regards, John

 

[EFLImpudence]

Could the test button electronics be so arranged to only trip if the earth was 'adequate'?