RCDs in series

Luckysasha

Anything wrong with this scenario. 3x6 Cable RCD protected at fuseboard ran to a shed feeding 2 x double sockets with built in RCDs. The cable is swa direct buried across a driveway

Tuco88

Ideally you should have an S-Type (Time Delay) Rcd upstream at the DB and Rcbos in the shed.

Nothing wrong with normal RCDs in series in such. Issue is if a Scokets circuit trips it will take out all the power to the shed.

2011

It is best avoided in my opinion. If practical I would would not have the cable supplying the shed in an RCD.

2011

2011 wrote: »

It is best avoided in my opinion.

Why do you say that 2011?

2011

Sir Liamalot wrote: »

Why do you say that 2011?

A few reasons:

1) If I have a less sensitive RCD in series with a more sensitive RCD what will this achieve?

2) It is not a regulation.

3) Less chance of nuisance tripping.

4) I assume that the RCD in the main board is for the sockets in the house. So if the shed trips the RCD all of the sockets in the house will be lost too. Shed sockets are more likely to trip the RCD if 110V transformers and power tools are used.

5) Quicker to fault find.

6) Tripping this RCD would mean loss of lighting in the shed which has its own risks.

Do it right
Install a sub-distribution board in the shed with an RCBO of the shed sockets and an MCB for the shed lights.
This avoids terminating a 6 sq. cable in the sockets or joining this cable.

All good points.

I cascade them when feeding distribution from multiple sources.
eg. 60mA inverter output, 30mA distribution.

Does the sub-distribution not have an upstream RCD in your scenario?

2011

Sir Liamalot wrote: »

Does the sub-distribution not have an upstream RCD in your scenario?

Nope.

So that's an unprotected conductor running the length of a premises?

2011

Sir Liamalot wrote: »

So that's an unprotected conductor running the length of a premises?

Unprotected? No it has an MCB. It is not normal to feed a sub-distribution board from an RCD.

Seems odd to me to put the only ELCB in the middle of a circuit.

2011

Sir Liamalot wrote: »

Seems odd to me to put the only ELCB in the middle of a circuit.

What do you mean?
What is odd??

Normaly RCDs are fitted to final circuits or a group of final circuits in the board they are fed from.
Supplies to sub-distribution boards are not normally fed from RCDs.

If I understand correctly you are saying it's; Incomer -> Isolator -> MCB -> RCD @ sub-distro.

What if somebody drills the live en route to sub-distro?

2011

Sir Liamalot wrote: »

If I understand correctly you are saying it's; Incommer -> Isolator -> MCB -> RCD @ sub-distro.

What if somebody drills the live en route to sub-distro?

You understand me correctly.

What if someone drills into the mains supply from the ESB meter to the distribution board?

This is why the regulations are based on risk assessments, statistics and experience. When any electrical cable is installed it should have sufficient mechanical protection in addition to overload, short circuit and sometimes earth leakage protection.

Most of the cables on an industrial installations that I deal with are not protected by RCDs and some of the LV stuff is at 690V and is fused at several hundred amps.

2011

I think that RCDs are a great invention, but there is no “silver bullet” when it comes to electrical safety. They only offer supplementary protection, have been known to fail and even when they work there is no guarantee that they will prevent electrocution.

Makes sense. I'll stick to cascading them and armour. Thanks for the feedback.

Tuco88

Was there not something about supply cable's to sheds/Outhouse must be Rcd protected. Cant remember was it a UK thing or is outdoor lightning under a particular hight?

Rcds offer some degree of fire protection also.

I would say an MCB or S/F supplying a small IP rated sub board in the shed with Rcbos for Sockets and MCBs for lights is the common way this is done.

2011

Tuco88 wrote: »

Was there not something about supply cable's to sheds/Outhouse must be Rcd protected. Cant remember was it a UK thing or is outdoor lightning under a particular hight?

I dont think so.
Besides some sheds are so big that there isn’t an RCD large enough.

2011

Sir Liamalot wrote: »

Makes sense. I'll stick to cascading them and armour. Thanks for the feedback.

If you want to have an RCD protecting the shed cable then my advice would be to supply it from a dedicated RCBO in the main distribution board. Why bother with additional RCDs in the shed??

Tuco88

Lets say 32amp Rcbo in the Distribution board supplying a small single row board with 16amp/10amp Mcbs so for sockets/Lights?

Sounds better.

2011

Tuco88 wrote: »

Lets say 32amp Rcbo in the Distribution board supplying a small single row board with 16amp/10amp Mcbs so for sockets/Lights?

Sounds better.

I won’t split hairs, it’s fine.
I couldn’t fault someone for doing it that way.
It’s just not the way I would do it.

Luckysasha

Sorry only getting back to this now. I have a separate 3 x 2.5 swa for the lighting circuit so no need to put a sub board in the shed. I was thinking the RCD on the main board would protect the cable up as far as the socket and then the in built rcd would take care of the socket itself. Maybe I’m just trying to over protect the 3 x 6 buried in the ground

Bruthal

Tuco88 wrote: »

Lets say 32amp Rcbo in the Distribution board supplying a small single row board with 16amp/10amp Mcbs so for sockets/Lights?

Sounds better.

Yea I would prefer an MCB protected cable out to the sub DB, and the sub DB then has individual circuits protected.

RCD in the circuit supplying the sub board and everything is gone with an rcd trip.

Bruthal

Tuco88 wrote: »

Was there not something about supply cable's to sheds/Outhouse must be Rcd protected. Cant remember was it a UK thing or is outdoor lightning under a particular hight?

Lights not attached to the building itself should be, I think it was.

Risteard81

As stated (assuming this isn't a TT installation) there is no reason to protect SWA cable with earthed armouring with an RCD. (With a TT an S-type (time-delayed) would be used with e.g. 100mA rated residual operating current.)

A switch fuse or MCCB is better to supply a submain than an MCB though as MCBs in series will not discriminate either when it comes to fault protection. That's why I don't like the requirement in the Rules for a 63A MCB in the Permalli Box over 3m now.

2011

Risteard81 wrote: »

A switch fuse or MCCB is better to supply a submain than an MCB though as MCBs in series will not discriminate either when it comes to fault protection.

Not quite.

If selected correctly MCB's can partially discriminate, i.e. it is not too difficult to achieve discrimination between MCBs for overload conditions.

Achieving full discrimination means including discrimination for short circuit conditions too. This is only achievable for a short circuit up to a certain point. In many cases the short circuit is sufficiently limited so total discrimination is achievable between MCBs once the ratings and characteristics are correctly selected.

Short circuit currents are limited by:
1) The rating of the upstream transformer
2) The impedance of the circuit.

This explains it quite well.

Bruthal

2011 wrote: »

In many cases the short circuit is sufficiently limited so total discrimination is achievable between MCBs once the ratings and characteristics are correctly selected.

Short circuit currents are limited by:
1) The rating of the upstream transformer
2) The impedance of the circuit.

Indeed, id say thats an accurate description. And number 2 there ties in with that,- the impedance of the circuit increases as you get to final circuits protected by smaller MCBs, so a short circuit is likely to be of a smaller magnitude the smaller the final circuit is, and so its more likely to trip the smaller MCB only.

A C type on the sub main would probably add a further level, with B on finals.

Risteard81

That's why I specified fault protection add opposed to overload protection.

Poulgorm

Slight variation on the thread: I use several electrical power tools from time to time in my garden - lawnmower, hedge trimmers and occasionally an electric rotavator. I use a 20 metre extension lead (fully uncoiled) and an RCD.

The extension lead is plugged in to a socket in the house. Where should I put the RCD? Should I put it in the socket in the house or at the end of the extension lead? Or does it matter?

Thanks, in advance.

Bruthal

Poulgorm wrote: »

Slight variation on the thread: I use several electrical power tools from time to time in my garden - lawnmower, hedge trimmers and occasionally an electric rotavator. I use a 20 metre extension lead (fully uncoiled) and an RCD.

The extension lead is plugged in to a socket in the house. Where should I put the RCD? Should I put it in the socket in the house or at the end of the extension lead? Or does it matter?

Thanks, in advance.

If the house has a properly working one in the board, there is not much point having on plugged in anywhere. Any fault tripping the plugged in one is likely to trip the main board one too.

But if using a plug in one anyway, at the start of the extension lead is better than at end of it.

2011

Remember RCDs can only protect what they feed, they can protect “upstream”.
As can be seen on this thread MCBs and RCDs can cause a lot of confusion. There are a lot of misconceptions out there!

Poulgorm

Bruthal wrote: »

If the house has a properly working one in the board, there is not much point having on plugged in anywhere. Any fault tripping the plugged in one is likely to trip the main board one too.

I don't have an RCD on the main board - Just an ELCB (plus MCBs + main switch).

I thought an ELCB was not effective if you are using a double insulated appliance (no earth wire) - and many electrical tools are like that.

Correct me if I am wrong! (I am not an electrician - as you may have guessed).