RCD on non-earthed supply

[Seb101]

I have a string of of festoon lights across the garden powered from a non-RCD protected lighting circuit. I like to have my Bluetooth speaker down the other end of the garden when I'm working, but recently it's battery is getting a bit old and it needs to be plugged in all the time.

So I built an extension cable with a 13A socket on one end and a bayonet cap light 'plug' on the other (not as a permanent installation, just for the occasions I'm using my speaker in the garden, which is a non-earthed appliance in any case). For added safety I put an inline RCD in the cable between the socket and the bayonet plug, in theory so any leakage current from the live to ground/moisture would trip the RCD. So I plugged it all in, and turned the light switch on but the RCD would not 'reset' it just stayed in the tripped mode. I realised I might have got the polarity wrong on the bayonet, so I carefully reversed the orientation of the bayonet connector and tried again. The RCD would still not reset. So I whipped the circuit tester out and tested the input terminals on the RCD, sure enough it was getting 240VAC at the correct polarity, but it simply refused to 'reset'.

So I took the cable and replaced the bayonet with a standard 13A plug and tried it indoors on an earthed circuit, sure enough it started working.

I have read that RCDs do not need an earth to operate, but is it possible the design of certain RCDs requires them to have an earth?

Bit confused by this one.

 

[securespark]

In order to get the RCD to work, what did you connect the CPC in the plug to?

 

[bernardgreen]

I have read that RCDs do not need an earth to operate,

They do not need an Earth connected to the RCD, ther is no Earth terminal on a normal domestic RCD ( RCBOs have a "functional Earth" to ensure they operate when the incoming Neutral is disconnected )

 

[Seb101]

In order to get the RCD to work, what did you connect the CPC in the plug to?

When it worked, after I put a regular 13A plug on it, it was connected to ground.

When it wasn't working with the bayonet cap, it was floating.

 

[securespark]

They do not need an Earth connected to the RCD, ther is no Earth terminal on a normal domestic RCD ( RCBOs have a "functional Earth" to ensure they operate when the incoming Neutral is disconnected )

MEM RCD's used to have a FE.

https://willrose-electrical.co.uk/wp-content/uploads/2015/05/A100METOLDCOLOURS-3.jpg

 

[ban-all-sheds]

You can absolutely see the point of an inline RCD tripping if it detects loss of earth.

 

[bernardgreen]

You can absolutely see the point of an inline RCD tripping if it detects loss of earth.

Do you mean tripping if it detects loss of Neutral ?

 

[ban-all-sheds]

No.

 

[magicmushroom666]

What RCD have you installed? The instructions probably mention the requirements for an earth connection

 

[bernardgreen]

When it worked, after I put a regular 13A plug on it, it was connected to ground.

What terminal on the RCD was connected to Ground via the 13 amp plug ?

 

[Seb101]

What RCD have you installed? The instructions probably mention the requirements for an earth connection

https://www.ffx.co.uk/tools/product...2727-In-Line-Active-Outdoor-Rcd-16A-230V-50Hz

I cannot find documentation for it (none was in the packaging).

 

[Seb101]

What terminal on the RCD was connected to Ground via the 13 amp plug ?

Ground/Earth... The RCD has L-N-E terminals on both the line and load end. It assumed the Earth was just a pass-through, but maybe it is not.

 

[EFLImpudence]

I have read that RCDs do not need an earth to operate, but is it possible the design of certain RCDs requires them to have an earth?

Yes.

 

[JohnW2]

They do not need an Earth connected to the RCD, ther is no Earth terminal on a normal domestic RCD

As I wrote a few minutes ago in another thread, many of the 'active' RCDs (in many RCD sockets, RCD FCUs and plug-in RCDs) will trip on loss of a connection to earth, and cannot be reset in the absence of an earth, and most/all of them use a connection to the CPC (if present) when the test button is pressed.

Hence some of those cannot be used at all without an adequate earth connection, whilst some can be used but will not trip when the test button is pressed.

Kind Regards, John

 

[bernardgreen]

My error. I was thinking it was a a normal domestic tpye RCD and not an Active RCD.