Time Delayed RCD Protection for a TT system

dc99 · Nov 8, 2012

Hello all

I have a standard CU at present with 30mA RCD protection. As there was no decent earth in place, I'm in the process of installing TT and, so far, I've installed an Earth Rod, tested and found sufficiently low loop impedance. My next job is to get the necessary RCD protection in place.

My (limited) understanding is that I need the normal 30mA RCD protection in place but also a 100mA Time Delayed RCD double pole to replace the main switch. What I'm not clear on is why I need a 100mA Type S RCD main switch if all circuits are already protected by 30mA RCDs or RCBOs. Is it belt and braces (in case the 30mA RCD fails), or is it in fact only required if you don't have full integrity already ?

My concern is that I have a Hager board and I'm struggling to find a Type S RCD that has low enough rating (Amps) that will fit it (my distributor's fuse is less than 100A, so I don't want to replace my main switch with a 100A RCD). I can find an 80A 100mA Type S RCD by MK, but that won't fit my Hager board and I don't want to replace the entire CU if I don't have to.

If the Type S 100mA RCD main switch is absolutely required, then my next question is... can I keep my CU as it is, but put a small board upstream of it which simply contains the Type S 100mA RCD? I.e. the incoming supply comes into the small CU, through the 100mA RCD, then straight out again and into the existing CU that has a standard (non-RCD) switch and 30mA RCD/RCBO protection on all circuits?

Sorry if this sounds long-winded :-/ Thanks in advance for any help you can offer.
Darren

ashrow · Nov 8, 2012

nothing wrong with fitting the 100A rated 100mA one and also nothing wrong with fitting one outside of the consumer unit

What happened to the original earth of the install, have you reported the lack of a decent earth reading ?

Engineer54 · Nov 8, 2012

The 100mA S type up-front RCD is a very prudent provision on any domestic TT system, especially one with, or around that numpty 200 ohm Ra. Yes it is there to provide addition protection to faulty/failed 30mA RCDs....

Is a 100A S type RCD that much more expensive than a 80A or 63A S type RCD then?? I thought that most CU main switch isolators were rated at 100 amp these days. All the S type RCD is, is an isolator switch (whatever it's CC rating is) with a delayed RCD, nothing more, nothing less!!!

And yes you can mount the S type upsteam in it's own enclosure to supply and protect your CU....

dc99 · Nov 8, 2012

OK - I was advised previously that if i have, say, a 60A distributors fuse, then I should have a 60A main switch (rather than 100A) so that a fault would trip my switch rather than the distributor's cutout.

Re the original earth, it seems there wasn't one. There's an earth clamp on the steel pipe that the cable is supplied through, but continuity is poor. UK Power Networks have been out to inspect, but said it was never installed by them and is the homeowners responsibility to get an earth (or pay for a PME upgrade, which in this case was prohibitively expensive), hence the TT.

Thanks for the info

Engineer54 · Nov 8, 2012

OK - I was advised previously that if i have, say, a 60A distributors fuse, then I should have a 60A main switch (rather than 100A) so that a fault would trip my switch rather than the distributor's cutout.

There is and never has been any protective elements in a CU's main switch (isolator). It's function is to isolate All circuits from the incoming supply. (single point of isolation)

So what are UK Power Networks wanting, to provide a PME connection?? If it's available on the connected network supplying the house then it should literary be peanuts in cost, and really should be FREE!!!

AndyL · Nov 8, 2012

dc99 said:
OK - I was advised previously that if i have, say, a 60A distributors fuse, then I should have a 60A main switch (rather than 100A) so that a fault would trip my switch rather than the distributor's cutout.

Re the original earth, it seems there wasn't one. There's an earth clamp on the steel pipe that the cable is supplied through, but continuity is poor. UK Power Networks have been out to inspect, but said it was never installed by them and is the homeowners responsibility to get an earth (or pay for a PME upgrade, which in this case was prohibitively expensive), hence the TT.

Thanks for the info

The 100A switch will never trip, it is a switch not a protective device. The rating lets you know what it is capable of carrying. You are actually better off with the switch being rated the same or higher than the main fuse

dc99 · Nov 8, 2012

Engineer54 said:
There is and never has been any protective elements in a CU's main switch (isolator). It's function is to isolate All circuits from the incoming supply. (single point of isolation)

So what are UK Power Networks wanting, to provide a PME connection?? If it's available on the connected network supplying the house then it should literary be peanuts in cost, and really should be FREE!!!

Never thought about the main switch like that - my 17th Ed instructor gave me that advice and I assumed at the time that it was so that the main switch would trip first. However, as you (and Andy L) say, it's not protective and won't trip, so that wouldn't make sense.

Re. UKPN, there is PME in the area, but my supply is very old and they've quoted me thousands to upgrade to it, to cover the cost of closing the road and digging it up, etc. ! I would love it to be a free upgrade (and have tried to convince many different people on the phone that they should do this) but they're adamant that my supply is so old that it never had a supplier's Earth, the Earth is the homeowner's responsibility and, whilst they'll happily give me PME, they'll charge me for the privilege.

Engineer54 · Nov 8, 2012

dc99 said:
Re. UKPN, there is PME in the area, but my supply is very old and they've quoted me thousands to upgrade to it, to cover the cost of closing the road and digging it up, etc. ! I would love it to be a free upgrade (and have tried to convince many different people on the phone that they should do this) but they're adamant that my supply is so old that it never had a supplier's Earth, the Earth is the homeowner's responsibility and, whilst they'll happily give me PME, they'll charge me for the privilege.

I don't understand that philosophy at all?? What's the age of the cable got to do with it!!!! If this cable is still in service and (from what they are saying) not suitable to provide your customs supply for whatever reason, then they should be providing a new cable anyway, under they're duty of maintaining their equipment!! Hell, ... what is your customer and everyone else paying the standing monthly installation charge for!! lol!!

Knobhead · Nov 8, 2012

dc99 said:
my 17th Ed instructor gave me that advice

That just about sums 17th instructors up. They'll teach you how to pass the exam after that.....

Rockingit · Nov 8, 2012

OP - here's some pointers for you:

1) Isolation (main) switch: Does just this, and nothing else. It turns you on, and it turns you off. The 100A stamped on the side is telling you the maximum current that it is designed to cope with in normal operation. You could have a 50A incoming fuse and a 400A main switch, it matters not (however, it would matter if it were the other way around..)

2) RCD - operates to break the circuit in the event of fault current leaking to earth. It is not an OverCurrent Protective Device (OCPD) - fuse or breaker to the average man. If you have a circuit which is under fault conditions but NOT leaking to earth (ie, a Live - Neutral short) it will do nothing. The numbers printed on it (ie 63A, 30mA) indicate firstly the maximum current it is designed to allow to pass through it in normal operation (as in 1 above), the second number is the amount of current that it needs to detect flowing to earth (as in a fault) in order to operate and break the circuit.

3) If you have a 100mA RCD and a 30mA in series in the same circuit, under fault conditions (normally assumed to be more than 100mA) it is anyone's guess as to which will trip first because both of them are sensing the same amount of fault current, over the threshold of both of them, so it comes down to which of them has the fastest response time.

4) Because of (3) we use a Time Delayed ('S' type) RCD as our 100mA - this means that it will pause a while before tripping, which means that the smaller 30mA RCD trips first - which is what we want.

5) This principle is known as 'discrimination'.

6) It really doesn't matter how the upstream 100mA RCD is fitted, and many end up in a separate 2U box.

Deleted member 9648 · Nov 8, 2012

dc99 said:
Hello all

I have a standard CU at present with 30mA RCD protection. As there was no decent earth in place, I'm in the process of installing TT and, so far, I've installed an Earth Rod, tested and found sufficiently low loop impedance. My next job is to get the necessary RCD protection in place.

My (limited) understanding is that I need the normal 30mA RCD protection in place but also a 100mA Time Delayed RCD double pole to replace the main switch. What I'm not clear on is why I need a 100mA Type S RCD main switch if all circuits are already protected by 30mA RCDs or RCBOs. Is it belt and braces (in case the 30mA RCD fails), or is it in fact only required if you don't have full integrity already ?

My concern is that I have a Hager board and I'm struggling to find a Type S RCD that has low enough rating (Amps) that will fit it (my distributor's fuse is less than 100A, so I don't want to replace my main switch with a 100A RCD). I can find an 80A 100mA Type S RCD by MK, but that won't fit my Hager board and I don't want to replace the entire CU if I don't have to.

If the Type S 100mA RCD main switch is absolutely required, then my next question is... can I keep my CU as it is, but put a small board upstream of it which simply contains the Type S 100mA RCD? I.e. the incoming supply comes into the small CU, through the 100mA RCD, then straight out again and into the existing CU that has a standard (non-RCD) switch and 30mA RCD/RCBO protection on all circuits?

Sorry if this sounds long-winded :-/ Thanks in advance for any help you can offer.
Darren

I think it's worth pointing out here...(perhaps not clear in the replies)....that there is no requirement for a 100ma S type RCD main switch/additional RCD as far as the 'regs' (bs7671) are concerned. As already pointed out though,it is prudent..as RCD's do fail,and on a typical UK TT system this may leave you with no earth fault protection without additional provision.