RCD protection for TT system

[fastJedi]

I have bought a Volex 12 way split CU with a 100A DP main switch and 63A 30mA RCD (in error!) as I now understand all circuits will need RCD protection. I am yet to prove Volex offer an RCD Incomer that will provide discrimination for the existing CU RCD and the Garage RCD (Either by trip current or time delay) Where the hell is the Volex web site???

If they do .... I am assuming the answer is to buy a 80A 'slow to operate' Incomer to replace the DP switch. If they don't what should I do?
a) Thow the Volex CU away and start again
b) Install an 80A / 100mA RCD in a separate plastic enclosure between the Meter and the CU.

The overall Earth impedence is 15 ohms (The spike on its own is 140 ohms)

 

[Lectrician]

See if the DNO will provide a PME (or even TN-S, depending on the service). Then use the CU you already have.

Make sure you upgrade your tails and main earthing conductor, and install the no-doubt missing main equipotential bonding to gas/water/oil etc.

Personally, I would through the volex away anyway......cheap ****e.

 

[Darkness]

've heard quite a few folk give off about Volex CU's being rubbish. Why is that anyhow? Is it the quality of the plastic etc or is there alot of probs with the MCB's and so forth?

I'm a Hager user btw.

 

[Darkness]

've heard quite a few folk give off about Volex CU's being rubbish. Why is that anyhow? Is it the quality of the plastic etc or is there alot of probs with the MCB's and so forth?

I'm a Hager user btw.

 

[Lectrician]

It's the enclosure.

Plastic crap.

Awful to work with.

Utter rubbish.



MCB's are not that much better - never seen one fail, admittadly, but they feel and look cheap.


MEM all the way for me.

 

[plugwash]

afaict they comply with british standards just about but they are cramped to work with and generally look and feel cheap.

 

[cozycats]

If as I understand you have a TT system then your main incomer rcd will probably be 100ma, therefore you will need the 30ma rcd for your socket circuits! If yor incomer rcd is only 30ma then you don't need an additional rcd, but investigate the pme option.WARNING unless your house has been rewired since 16th edition this could prove to be an expensive option, although it will all have to be upgraded eventually

 

[Big_Spark]

Lectrician has offered the best advice here, PME would be the best route to go, however if this is not an option I would suggest this;

Forget the Volex, for the reasons highlighted.

Buy a MEM MEMshieldII Insulated Enclosure with the correct number of ways required.

Buy a 100A 300mA Type S (Time Delayed) RCD Main Incomer

Buy Type B MCB's of the appropriate rating for your Lighting circuits

Buy appropriately rated 30mA RCBO's for all Power Circuits (Sockets, Immersion etc)

Hager do some good kit too, but my personal preference is MEM for domestic and single phase stuff.

 

[BSBS]

Wouldnt a 300ma be a little to big given Regulation 413-02-20

Ra * I delta n < 50V

140 * .30 = 42V

Now i know it is below 50V but it is winter come the summer when it dries out more your earth path resistance could increase pushing it above 50V..
So your maximum Earth electrode resistance will be

50 / .30 = 166ohms

Also only outdoor equipment and sockets reasonably expected to be used for outdoor use need to be on a 30ma RCD so i really cant see why you would want to put yout immersion heater on it it's just going to cause a lot of nusiance tripping further down the line..

 

[plugwash]

on a TT all sockets need to be on 30ma rcd

 

[BSBS]

No they do not

Regulation 471-08-06 (Residual Current devices for socket outlet circuits in a TT system, index page 317)

"Where the measure is used in an installation forming part of a TT system, every socket-outlet circuit shall be protected by a residual current device and shall comply with regulation 413-02-16"

Regulation 413-02-16

"If Protection is provided by a residual current device the following condition shall be fulfilled: Zs * I delta n < 50v"

Page 21 fig 3a OSG ( Installing RCD's in a TT installation)

30ma RCD Note 2

"circuits to portable equipment outdoors and socket outlet circuits that may reasonably supply portable equipment outdoors (ground floor sockets)"


I was always led to belive that All sockets should be on a 30ma RCD however, I do still and will continue to on new installations put the socket circuits on the 30ma side the point is that it is not a requirement of the regulations as long as 413-02-06 is met and it is not reasonable expected to be used for outdoor equipment then you have complied with the regulations.

 

[Big_Spark]

Wouldnt a 300ma be a little to big given Regulation 413-02-20

Ra * I delta n < 50V

140 * .30 = 42V

Now i know it is below 50V but it is winter come the summer when it dries out more your earth path resistance could increase pushing it above 50V..
So your maximum Earth electrode resistance will be

50 / .30 = 166ohms

Also only outdoor equipment and sockets reasonably expected to be used for outdoor use need to be on a 30ma RCD so i really cant see why you would want to put yout immersion heater on it it's just going to cause a lot of nusiance tripping further down the line..

Why would it..you need to have proper discrimination for the circuits protected by the RCBO's.

Installers who blindly stick a bog standard 30mA RCD as the main switch on an entire installation are fools. They are misinterpreting the regulations and they are not considering every aspect.

Further, the regulations state as quoted above that ""circuits to portable equipment outdoors and socket outlet circuits that may reasonably supply portable equipment outdoors (ground floor sockets)" so those installers who also blindly put RCD's on ALL socket circuits WITHOUT good cause are also no designing the installation to comply with either common sense or the spirit of the Regulations.

However TT systems are a different matter, special precautions need to be considered due to the increased risk of shock under fault conditions due to the nature of the Earthing arrangements..however TT systems kill and injure no more people each year than any other type of service. For this reason the Main incomer should always be a time delayed RCD of no less than 300mA. Then all subsequent circuits, except lights, fire and burglar alarms, should be protected by 30mA RCBO's of the appropriate rating. This allows proper discrimination of the circuits under fault conditions, but it also ensures that should a fault occur on one of the MCB circuits, the Main Incomer will still protect them if it is an Earth Fault Current.

 

[ban-all-sheds]

Why would it..you need to have proper discrimination for the circuits protected by the RCBO's.

Wasn't his argument that when the immersion heater gets a bit leaky in years to come it will start to trip a 30mA RCBO, and it doesn't need to be on a 30mA one in the first place?

Installers who blindly stick a bog standard 30mA RCD as the main switch on an entire installation are fools. They are misinterpreting the regulations and they are not considering every aspect.

Again, if I understood BSBS, he was querying the use of a 300 instead of the more normal 100, not a 30.

Then all subsequent circuits, except lights, fire and burglar alarms, should be protected by 30mA RCBO's of the appropriate rating. This allows proper discrimination of the circuits under fault conditions, but it also ensures that should a fault occur on one of the MCB circuits, the Main Incomer will still protect them if it is an Earth Fault Current.

You need to use DP RCBOs, though, as SP ones won't clear a N-E fault, and shortly after an SP one fires, the main incomer will trip as well.

 

[Big_Spark]

BAS..you dont not know how the kit works, thast is obvious, single Pole RCBO's are IDENTICAL in their operation to double pole RCBO's..and please don't try to argue with me as on this point I will win the arguement..I appreciate that many people may not think they are the same, but they are...Single Pole RCBO's have the circuit PHASE and NEUTRAL connected to it, then a built in fly lead connects the RCBO to the Neutral Bar, and a fly lead connects it to the Earth Bar so that it is able measure the relationship between the Phase and the Neutral and Earth.

As for your comment about an immersion heater..that is the entire point of putting it on an RCBO when you have an RCD incomer, if you don't it will eventually disconnect the whole supply to the board..thus leaving you with not lights, no power and up shiite creek if the main incomer will not reset until the fault is cleared.

Again, if I understood BSBS, he was querying the use of a 300 instead of the more normal 100, not a 30.

He didn't actually state a figure and whilst I take your point, the 300mA Type S gives better discrimination characteristics to avoid nuisance tripping of the main incomer. 100mA Type S RCD incomers can sometimes be so good that a fault on a subcircuit can bypass a 30mA RCBO and trip the incomer..something we want to avoid.

 

[Adam_151]

I believe BAS's point about single pole RCBOs was that they function the same as double pole ones, except that when they trip, they only cut phase and not neutral. So a Neutral -> earth fault that causes the RCBO to trip will not be isolated, and so will remain in place and trip the time delay RCD incommer a small time later

Edit: sorry, wrote "phase->earth fault" instead of "Neutral -> earth fault". Now corrected