Correct Type-S Upfront RCD for 3Ph TT System

[Risteard]

All fusebox RCBOs are now switched neutral. I don't think they make any other kind now.

Only the compact ones.

 

[suffolkspark]

So I will use something like this (found a type A time delay) which I will put in a separate enclosure to replace that 100A isolator.

100A 4 Pole 100mA Type A Time Delay RCD

Test button

www.cef.co.uk

If those fusebox rcbos aren't switched neutral your gonna have a problem....

The tails appear to all be visible there so I wouldn't add upfront rcd protection to anything other than the submain going off to the kitchen.

 

[westward10]

If the supply conductors enter an all insulated enclosure then fault protection is not required.

 

[Raptor0014]

If those fusebox rcbos aren't switched neutral your gonna have a problem....

The tails appear to all be visible there so I wouldn't add upfront rcd protection to anything other than the submain going off to the kitchen.

They are definitely DP as they're the mini ones.

I'm a little confused as to why you say a upfront for the whole system isn't required? I was led to believe when I did my course (granted I think 'guru graham' as I used to call him was good at passing off his thoughts as a regulation sometimes) that on a TT it must have upfront RCD to protect the tails regardless.

I need to make sure this one has no hiccups as it will be looked at by my CPS assessor so that I can pay for the privilege of being a member for another year....

So I am not questioning your judgement. I just need to know how that fits into the regs as my understanding is its a no ifs, buts or maybes. But could be the way I've had it explained.

 

[westward10]

Why do you think they need rcd protection.

 

[Raptor0014]

The way I had it explained was that if a fault occurred between the tails and the board then it wouldn't be picked up by the RCD's in the board.

 

[westward10]

What type of fault.

 

[nicebutdim]

The full height are

Just did so quick research and can find nothing to suggest Fusebox have made full height RCBOs that switch both poles. Their new AFDD RCBOs are tall and switch both poles.

 

[Raptor0014]

What type of fault.

Line to earth

 

[Raptor0014]

Just did so quick research and can find nothing to suggest Fusebox have made full height RCBOs that switch both poles. Their new AFDD RCBOs are tall and switch both poles.

Yeah I think I have gotten it confused with the mini ones and the AFDD ones I was quoting for the other day (some bloke wanted all 14 circuits protected with AFDD! Soon changed his mind when I told him that was £1750 before we even started on the remaining materials and labour etc)

 

[westward10]

Line to earth

They would have to be in closed proximity to any earthed metalwork and be prone to damage from the metalwork but I assume they are not.

 

[Raptor0014]

They would have to be in closed proximity to any earthed metalwork but I assume they are not.

I'm only questioning it so I understand it. As said I have always been told that a TT system must have upfront time delay RCD.

If I can get away with using a cheaper DP time delay then that is better for the client. I just want to have the right answer in my head if Mr CPS assessor asks why I have done it as he thinks it's not compliant.

 

[westward10]

There is no requirement in BS7671 for an rcd to provide fault protection however it is the preferred method. The tails are installed in such a manner that ADS is not necessary and to be honest I doubt the assessor will comment on it. Concentrate more on the theory for ADS for your final circuits where a TT system is employed.

 

[pc1966]

As @westward10 says the requirement for ADS is independent of how it is achieved

TT systems have a shorter disconnect time requirement than TN as any metalwork normally goes to more or less full line voltage during a fault, where as TN is usually around half line voltage. That higher voltage is due to the Ze for a TT system typically being many ohms, so not much current flows and so the line is not "pulled down" much during the fault.

And it is this lack of a high PFC that makes meeting ADS difficult to impossible without the use of an RCD, as often the PFC could be less than the normal OCPD rating!

The second point is the risk of a fault needing ADS.

In the case of plastic enclosures (which are no longer done for domestic due to the fire aspect) there is very little risk of a tail shorting to metalwork, so they probably don't need protection. Even with a metal CU if the tails enter via a proper gland and are correctly terminated and tightened the risk could be argued to be very small (even more so if the 19-strand flexible tails are used so less forces on the main switch, etc)

So if the final circuits are all on RCD protection then you probably don't need the up-front RCD, though having one has the slight advantage of no single point of failure (i.e. a failed/stuck final RCD/RCBO would not cause all CPC to go live as the incomer would trip after ~0.2s).

A SWA sub-main is altogether different as a fault there is very likely to result in such a short and then without up-front RCD very unlikely to disconnect!

Traditionally (i.e. in the days before the 30mA additional protection for shock was common) a TT system would have a RCD incomer (or before that, a VOELCB) as the sole means of meeting ADS due to the high cost of the RCD. And to keep nuisance tripping down it would often be 100mA rated and possibly with a delay. These days practically all domestic circuits need 30mA RCD so that is no longer common.

But if you do need an up-front RCD(s) for any reason then to achieve selectivity you need to meet several conditions:

• The trip current should be around x3 higher (or more) so a low level fault still is sure to disconnect on the down stream one (as typically RCD trip somewhere between 0.5 and 1 In). E.g. 30mA would trip before a 100mA rated one reached 50mA where it might decide to go.
• The upstream one should take about 0.2s longer then the next one down, so under high fault current conditions (where all RCD have trip level exceeded) the one closest to the fault can disconnect before the up-stream ones decide to act.
• To avoid a N-E fault tripping out the upstream ones, the downstream has to disconnect N as well. Traditionally RCD always do that, but until recently few RCBO would do so.

Few systems have more than two in cascade, so typically you see 100mA or 300mA "S" selective ones used, but if you go to bigger systems you get adjustable MCCB ones where you can dial in the trip current and the trip times to suit the overall system design if 3 or more are used in series.

 

[Raptor0014]

As @westward10 says the requirement for ADS is independent of how it is achieved

TT systems have a shorter disconnect time requirement than TN as any metalwork normally goes to more or less full line voltage during a fault, where as TN is usually around half line voltage. That higher voltage is due to the Ze for a TT system typically being many ohms, so not much current flows and so the line is not "pulled down" much during the fault.

And it is this lack of a high PFC that makes meeting ADS difficult to impossible without the use of an RCD, as often the PFC could be less than the normal OCPD rating!

The second point is the risk of a fault needing ADS.

In the case of plastic enclosures (which are no longer done for domestic due to the fire aspect) there is very little risk of a tail shorting to metalwork, so they probably don't need protection. Even with a metal CU if the tails enter via a proper gland and are correctly terminated and tightened the risk could be argued to be very small (even more so if the 19-strand flexible tails are used so less forces on the main switch, etc)

So if the final circuits are all on RCD protection then you probably don't need the up-front RCD, though having one has the slight advantage of no single point of failure (i.e. a failed/stuck final RCD/RCBO would not cause all CPC to go live as the incomer would trip after ~0.2s).

A SWA sub-main is altogether different as a fault there is very likely to result in such a short and then without up-front RCD very unlikely to disconnect!

Traditionally (i.e. in the days before the 30mA additional protection for shock was common) a TT system would have a RCD incomer (or before that, a VOELCB) as the sole means of meeting ADS due to the high cost of the RCD. And to keep nuisance tripping down it would often be 100mA rated and possibly with a delay. These days practically all domestic circuits need 30mA RCD so that is no longer common.

But if you do need an up-front RCD(s) for any reason then to achieve selectivity you need to meet several conditions:

• The trip current should be around x3 higher (or more) so a low level fault still is sure to disconnect on the down stream one (as typically RCD trip somewhere between 0.5 and 1 In). E.g. 30mA would trip before a 100mA rated one reached 50mA where it might decide to go.
• The upstream one should take about 0.2s longer then the next one down, so under high fault current conditions (where all RCD have trip level exceeded) the one closest to the fault can disconnect before the up-stream ones decide to act.
• To avoid a N-E fault tripping out the upstream ones, the downstream has to disconnect N as well. Traditionally RCD always do that, but until recently few RCBO would do so.

Few systems have more than two in cascade, so typically you see 100mA or 300mA "S" selective ones used, but if you go to bigger systems you get adjustable MCCB ones where you can dial in the trip current and the trip times to suit the overall system design if 3 or more are used in series.

Thank you for the detailed explanation. Very much appreciated.

This is in a commercial setting hence I wanted to be doubly sure I had ‘all my ducks in a row’.

I knew some of the points regarding the use, 3x higher rated etc. But never really had the issue explained like the way you have. So thank you.

Makes things a lot clearer for me now so will go with the suggestion by Suffolkspark and replace the fused isolator only with a new enclosure and 100mA time delayed DP RCD.

 

[JosephandAmber]

Sorry to hijack this thread but in a similar situation. We have a TT three-phase supply, going into 5 switched fuses that feed 4 flats + landlord supply via VIR cables in a metal conduit. All final circuits are RCBOs.

Should I insist that an upfront RCD is added? If so, how would you configure this? Individual RCDs for each set of tails or a 4 pole protecting the lot? Something like... 100A 4 Pole 100mA Type A Time Delay RCD - https://www.cef.co.uk/catalogue/products/4945628-100a-4-pole-100ma-type-a-time-delay-rcd

 

[pc1966]

You do need a RCD before anything with metalwork that might come in contact with the supply, otherwise it won't disconnect!

The Hager one is a good choice, assuming your 3-phase supply is 100A fused, but I would go for a 300mA delay version so you can allow 150mA leakage from all 5 flats = 30mA each which is easy to approach to with a few circuits with electronics on them (including modern white goods with CPU control, etc).

Edit, here:

100A 4 Pole 300mA Type A Time Delay RCD

All RCDs have trip free mechanisms and can be padlocked either On or Off

www.cef.co.uk

 

[pc1966]

Also to add that with such an RCD your earth rod arrangement should be comfortably below an Ra of 167 ohms (50V / 0.3A) rather than the 200 ohm typically assumed for the more common 30mA/100mA RCDs.

 

[JosephandAmber]

I assume that in my instance it is okay to have the RCD before the switched fuses given some of the phases have been split with henleys to provide 5 supplies?

 

[pc1966]

I assume that in my instance it is okay to have the RCD before the switched fuses given some of the phases have been split with henleys to provide 5 supplies?

Yes, so long as the RCD has some form of 100A limiting there such as from the DNO fuses, or sum of down-stream OCPD.