additional RCD protection from old Wylex fuse oard

[haptism]

My mate has an old wylex board with the push in rewireable fuses; and has asked about fitting an rcd to one circuit (a ring main) that supplies one outside socket (at the mo he's using a plug in portable rcd when using the mower). He does not want a new consumer unit, and i know that rcbo's are not compatible with these old fuseboards so thats not an option; a rcd spur to the outside socket is not an option either because that means taking out the kitchen sink to access the ring where it loops outside. Therefore he wants to protect the whole circuit with an rcd located near the fuseboard.

Could I take one leg of the ring via the rcd and leave the other leg in the fuseboard, or would both legs of the ring need to go through the rcd ?

Thanks

 

[Darkwood]

haptism - I think you need to review your knowledge of how a rcd works and how current flows in a ring main...

A rcd as rightly mentioned will operate when their is an imbalance of a predetermined value to trip the device, that is 'what goes down the L must return back down the N' so if this deviates approaching the devices rating then the device will trip.

Now for a ring main - when you plug an appliance into a ring the current that flows to and from that item and will distribute down both legs of the ring - the balance of the distribution will be determined by several factors so if you were to have the RCD only looking at one leg of the ring it cannot monitor the full circuit, a dangerous scenario for anyone relying on the device to save their life, I would expect if you did try this set-up it would just constantly trip as current flowing though the L and N will be subject to the effects of terminal tightness, temp' etc where even a minor variation could see the N current differ from that of the L through the RCD as it returns down the other side of the leg where you have no rcd monitoring it.

If you want to fit a rcd to cover the ring then your ring main (both legs) need to be terminated to the load side of the rcd.

 

[davesparks]

When the RCD trips the circuit will remain live on both sides of the RCD. So nobody will know that it has tripped.
If it has tripped because someone has received an electric shock then it won't prevent their death as intended. But that person knowing their RCD has been installed may have a false sense of security from this.
Also the operation of the RCD will have effectively made the circuit into one big 32A radial in 2.5mm cable, this the cable may well become overloaded with the potential to start a fire.

So hey, crack on! It's only one dead person and their house burnt to the ground, I'm sure your insurance will cover it and you'll get a free holiday at her majesty's pleasure

 

[Dave OCD]

I was going to say similar but I knew Dave would be along at some point to save me the job. :yesnod:

 

[davesparks]

It's my bright and cherry outlook on life

 

[Marvo]

I'm going to close this thread for two reasons, firstly it's turning into a step by step guide on designing a specific circuit for an ongoing job at a customers premises which isn't what the forum is about and secondly, from the discussions above, I believe there's a chance of a hazardous outcome.

Please don't take this the wrong way Haptism when I suggest you get some on-site assistance with this even if only to come up with a practical and safe design.