100ma or 30ma??????????

[mickytuff]

Hi Everyone, a quick question for you all, if on discovering a property had no main incoming earth, there were no earth block (tnc-s) to connect off either, you installed an earth rod and the reading was acceptable below 200 ohms, i think, what would you use on your fuseboard for your main switch a 100ma/100a rcd, and then 2x other 30ma/63a rcd's to split the circuits up accordingly, just wondered as a 30ma main rcd would not realy be aproite as u'd loose all supplies to circuits, any help would be topnotch

 

[rocker]

I'd use a seperate 100mA incomer (in a split in the tails), and use a normal main switch and 2X 30mA RCDs, as the 100mA RCD is unlikely to give double pole isolation. You're right about not using another 30mA as the incomer, this would be overkill and it would be pointless having a 17th ed board with the other 2 RCDs in it!

EDIT: Chriist knows what happened there, sorry about the gibberish!

 

[mickytuff]

i sort of understand your answer, the bit bout splitting the tails and putting it in, not sure why?, could you explain a bit more please??as i cant see the benefit unless purely for the isolating purposes???

 

[johnboy6083]

100ma will not discriminate with 30ma (unless S types used), as most faults will be over 100ma anyway, normal plastic dual RCD board should do the trick.

 

[rocker]

The thing is, you need to have a double pole isolator - your main switch in a consumer unit... RCDs usually only isolate the Line side. So for this reason, you shouldn't put the 100mA RCD in place of the main switch in the consumer unit. So, you put the 100mA in a modular enclosure on the tails, i.e. the tails come from the service head, to the 100mA, and then to the board.

Using a 100mA RCD is not actually required on all TT systems, but for ease, convenience and added safety I always install them on such a system.

 

[Hawk81]

I'd use a seperate 100mA incomer (in a split in the tails), and use a normal main switch and 2X 30mA RCDs, as the 100mA RCD is unlikely to give double pole isolation. You're right about not using another 30mA as the incomer, this would be overkill and it would be pointless having a 17th ed board with the other 2 RCDs in it!

EDIT: Chriist knows what happened there, sorry about the gibberish!

Not sure why you think the 100mA rcd will not give double pole isolation? I would use a 100mA time delayed RCD in this case.

 

[mickytuff]

i understand the purpose of using a 100ma before the 30ma (to prevent loosing all circuits), i meant the bit bout the location of the 100ma rcd???

 

[tonys]

I would get in touch with the supplier and ask if they would turn it into a pme system.

 

[rocker]

What of it? I don't know if I understand what you're asking. One of the purposes of the 100mA RCD is to protect from faults to the tails and (metal) consumer unit, that's the reason it is usually located in it's own enclosure, before the consumer unit. That's how I understand it, anyway.

 

[johnboy6083]

IMO, theres no need for a 100ma RCD (S type or not), unless you are protecting a submain to an outbuilding, in which case, to acheive correct discrimination, you would need an S type. RCD's are used in TT systems to make up for the fact that it is nearly impossible to acheive the correct Zs without them, well that was before the regs about equipotential zones, (16TH EDITION), and now zones and protection of cables, which require RCD's for nearly all circuits anyway (within domestic installation)

 

[rocker]

Good point.

 

[mickytuff]

thanks for that rocker, now i get the full picture, depends realy whether you can squeeze a big board in or not!!, hence the plan b option and connect it via the tails, top advice, query answered

 

[Deleted member 9648]

Standard 100A DP main incomer and 2 RCD split in an insulated DB....any ways on the main switch before the RCD's can be used with RCBO's.....even better 100a DP main incomer and RCBO's all the way,insulated DB of course.

 

[mickytuff]

so are you saying that theres no need for 100ma rcd in this TT system?????? (wirepuller)

as others are saying differently, confused again????

 

[Deleted member 9648]

If all final circuits are fed from an insulated DB and are RCD protected within the DB what do you want an up-front 100ma RCD for?....the DB is all insulated so there is no need to provide earth fault protection to the tails/enclosure.