RCD sockets in the Garage? Or Plug in RCD?

[dishman]

I need to replace some sockets in the garage. Currently they are surface mounted plastic sockets. One is a single socket which the plastic pattress box has cracked. The other is a double socket which has come away from the wall (It could just be re-attached, altho an indicator light has broken).

I guess my options are.

1) Replace with two double surface mounted plastic sockets. Using a plug in rcd for any tools (may not be good for a garage, easily broken?)
2) Replace with two double metal clad surface mounted socket. Using a plug in rcd for any tools.
3) Replace with two active RCD medal clad surface mounted double sockets

I have gone for active ones as being the safest for tools.

I have read on this site that an RCD may be considered unnecessary to be combined within the socket for a garage, or at least, a bit over the top. I also read that an active rcd socket could trip the RCD on the consumer unit. I do not know if either of these opinions/statements are true.

Are there any downsides I am not aware of?

So, which would people consider the preferential option.

This is the RCD I was considering. They look to be a reputable company. The products is a lot cheaper then the MK variety though.

http://www.timeguard.com/products/s...ion-bulk-range/tfa07m-rcd-double-metal-socket

The trip speed is 40ms however. So the one below may be a better option.

https://www.tlc-direct.co.uk/Products/CM4901M.html

Many thanks in advance.

 

[John D v2.0]

If you have an RCD at the CU covering that circuit, check its rating. If it's already 30mA, then your extra ones won't be doing anything other than providing redundancy.
For discrimination you could have a faster one with 10mA sensitivity, but it's not the norm.
I'd just fit a normal socket, unless it was likely to get bashed in which case normal metal clad would be more appropriate.

 

[Taylortwocities]

I agree with John. First of all, if you already have a 30mA RCD in your house consumer unit for that circuit, then you do not need more in the garage!

If you need to protect the garage sockets (ie no RCD in the house) then you could have an RCD in feed from the house protecting all of the sockets. Is there a mini consumer unit in the garage? or does the feed from the house just go into the garage sockets.

Do not be confused about trip times. The devices all comply with the relavent standards. RCDs are made to those standards. 40ms is the standard trip time for a 30mA RCD when presented with an earth fault current of 5 x the rated current. That is the standard. So that Timeguard socket will not trip any faster than A N Other socket!

 

[JohnW2]

For discrimination you could have a faster one with 10mA sensitivity, but it's not the norm.

... and nor would it be by any means guaranteed way of achieving discrimination.

Kind Regards, John

 

[dishman]

Whoopse I got mixed up between mA and mS when comparing RCD's.

Yes the garage sockets are protected by a 30mA RCD on the consumer unit. It covers all sockets, the shower and the cooker. The MCB for the garage (and under-stairs area) is 16amps and it powers 6 sockets two doubles and 2 singles (one double and 1 single is under the stairs).

Even though it was already protected, I was not sure if it was best practice to have some form of RCD on the socket in use (either in the socket or plug in type) to prevent, in the situation it did trip, all the sockets in the house going off.

Is there any downsides to having an RCD (in the socket or plug-in type) on a circuit already protected by an RCD in the consumer unit?

Maybe as you say, I should stick with just two plastic doubles.

 

[flameport]

Is there any downsides to having an RCD (in the socket or plug-in type) on a circuit already protected by an RCD in the consumer unit?

Some types will cause the main RCD to trip when the test button is pressed, and as you should press the test button on RCD sockets and plug in adaptors before using them, that would be rather inconvenient.
You would also be paying for additional RCDs which are not required.

 

[Taylortwocities]

Is there any downsides to having an RCD (in the socket or plug-in type) on a circuit already protected by an RCD in the consumer unit?

No real downsides, just a bit pointless. If there's an earth fault then there is no guessing which RCD will trip. Sometimes both will trip.
Also, when doing the regular test procedure (pressing the test button) then both might trip.

 

[JohnW2]

....I was not sure if it was best practice to have some form of RCD on the socket in use (either in the socket or plug in type) to prevent, in the situation it did trip, all the sockets in the house going off.

The problem there is that it is by no means guaranteed to work light that. If you have two (say one in the CU and another in the socket, or plug-in) then, in the event of a fault, just one of them (either one of them), or both, may trip. Hence a fault in the garage might well kill all the sockets in the house if it is fed from the same RCD (in the CU) as the house sockets. The only way to avoid that is to not have RCD protection (or, at least, not the same RCD as other house circuits) at the house end of the garage supply.

Is there any downsides to having an RCD (in the socket or plug-in type) on a circuit already protected by an RCD in the consumer unit?

Only the possible inconvenience of having to reset both RCDs if both tripped as a result of a fault.

Kind Regards, John

 

[JohnD]

you mention RCD adaptors, sometimes still found in garden centres. In my (limited) experience I have found these to be less reliable and have a short life. On pulling apart I thought the components rather flimsy.

in the days when RCD sockets were more common, I handled a number of them. One was faulty out of the box and the makers sent me a replacement next day, no quibble, though they wanted the batch number and date off the internal sticker. IIRC it was a Greenbrook Powerbreaker.

It is more convenient to have the RCD near the point of use so you can quickly reset it. Sometimes you will hear it tick off, which may alert you to the cause. If for some reason you feel the need to have an RCD at origin and also at point of use, you can get time-delayed RCDs, but the circuit needs to be designed correctly.

 

[ericmark]

As said no real need to have a 30 mA at 40 mS RCD duplicated. The norm is you jump by factor of 3, so you have a 100 mA RCD feeding 30 mA RCD's and 30 mA feeding 10 mA. But with domestic no real point accept.

An active RCD will drop out with a power failure. So nothing to do with being a RCD it could be a simple contactor but because it drops out that means if the house RCD trips while your using a power tool and you go back to house to reset, then the power tool will not auto restart.

Since most power tools have a auto return to off switch with most this is not required, but on the odd one where it will not auto return to off using active RCD plugs is a good safety measure simply to stop auto restart.

I fitted active RCD FCU to our grinders to make them comply with auto off when power fails. Cheaper and simpler than using contactor.

 

[Taylortwocities]

As said no real need to have a 30 mA at 40 mS RCD duplicated. The norm is you jump by factor of 3, so you have a 100 mA RCD feeding 30 mA RCD's and 30 mA feeding 10 mA. But with domestic no real point accept.

Sorry Eric. That is not the case. You cannot discriminate RCDs by using CURRENT (like you do with MCBs). You must have TIME as the discrimination factor.

Think about it. Lets say you have a string of RCDs 100mA at the source, 30mA in the middle and a 10mA at the load end.
Granted that a small earth fault on the load end of say 20mA will take out the 10mA, and not the others, BUT this rarely happens,
What you usually get is a socking great big earth fault!

In that case probably all three RCDs will trip, maybe just the one that goes is the one with the quickest trip time, could be any one of them.

That's why we use a time-delay type S RCD, typically at the source of TT installations.

 

[securespark]

Our old friends the NIC used to (do they still?) suggest using two RCD's for appliances used outside the house (typically a hardwired device protecting the outlet and another on the lead) on the basis that if one fails, the other will provide protection.

I did suggest that any RCD could fail, leaving no protection (eg shower circuit) but apparently it is more important to "double up" on portable appliances outside as this is the greatest risk??

 

[Taylortwocities]

Hmm. Do they suggest having two MCBs on each circuit too

 

[EFLImpudence]

Best have three, just to be safe.

 

[JohnW2]

Our old friends the NIC used to (do they still?) suggest using two RCD's for appliances used outside the house (typically a hardwired device protecting the outlet and another on the lead) on the basis that if one fails, the other will provide protection.

As has often been said, the one (and only) advantage of having two (or more!) RCDs protecting a circuit is that of redundancy - sometimes supported by the (IMO very iffy) oft-cited 'evidence' that the failure rate of in-service RCDs is quite high.

However, particularly these days, I do not personally see that the 'importance' of RCD protection (hence, in some people's minds, the argument for redundancy) is any greater with "appliances to be used outside the house". Now (and for quite a long time) the majority of such equipment is Class II, and many/most only have 2-core flex. Hence, about the only scenario in which an RCD would normally be of any use at all would be if the flex were cut, the OPD did not operate and someone picked up the 'live' end of the cut cable whist being in contact with the ground.

If they were actually thinking (no guarantee!), I suspect that the requirement for RCD protection of sockets 'likely to supply outdoor equipment' arose in the days when Class I outdoor equipment (and/or 3-core cables) was far more common.

Kind Regards, John