EICR 100ma RCD TT systems

[Storm93]

Good Morning all ,

Carrying out an EICR yesterday on a domestic property TT system , which is not a system I frequently work on. My question is that I have a Plastic CU, All circuits protected by RCBO’s, Satisfactory main protective bonding in place & A Good ZE / PFC well within the limits of a TT system, however no 100ma fault protection at or before the CU at any point. What Code would this warrant?

Thanks in advance!

 

[westward10]

Does it require fault protection.

 

[Storm93]

Does it require fault protection.

As far as I can tell I am trying to determine if it needs the 100ma for fault protection which is where I’m stuck , it’s within a domestic house , so the meter tail run is a short run , the consumer unit is plastic so it cannot become live, and all final circuits are protected with RCBO’s, so this is where I’m not sure on the coding of such an installation?

Does it require fault protection.

 

[westward10]

What are you seeing as the danger, all insulated consumer unit and pvc/pvc supply conductors. What is going to create a fault condition.

 

[Storm93]

What are you seeing as the danger, all insulated consumer unit and pvc/pvc supply conductors. What is going to create a fault condition.

 

[Storm93]

That’s very true , My ZE reading obtained was 6.20 ohms , Calculating using ohms law would give me 37 amps, under fault conditions that would not blow a 60 amp fuse within 5 seconds , and could potentially leave the metal work live, would this warrant the code of a C2 on it?

Thanks for your help !

 

[loz2754]

the meter tail run is a short run , the consumer unit is plastic so it cannot become live, and all final circuits are protected with RCBO’s

You've answered your own question. No code needed.

 

[loz2754]

That’s very true , My ZE reading obtained was 6.20 ohms , Calculating using ohms law would give me 37 amps, under fault conditions that would not blow a 60 amp fuse within 5 seconds , and could potentially leave the metal work live, would this warrant the code of a C2 on it?

Thanks for your help !

What fault conditions? All outgoing circuits are RCD protected.

 

[westward10]

That’s very true , My ZE reading obtained was 6.20 ohms , Calculating using ohms law would give me 37 amps, under fault conditions that would not blow a 60 amp fuse within 5 seconds , and could potentially leave the metal work live, would this warrant the code of a C2 on it?

Thanks for your help !

What metalwork is going to become live between the service head and consumer unit.

 

[Storm93]

What fault conditions? All outgoing circuits are RCD protected.

Okay I understand , one final question just for a future reference, say in a hypothetical scenario as per above but the tails were say in a containment system with protective main bonds, Through a wall, or something like, and there was a potential for those tails to be in contact with extraneous conductive parts or somewhere along the route an exposed conductive part (before a final circuit) , would this then raise it to a C2 code? Sorry I’m just making sure that I have a good understanding for future ha.

 

[Storm93]

What metalwork is going to become live between the service head and consumer unit.

Yes I understand, so if in a larger installation where there was a potential for this to happen for whatever reason , say between the head and the CCU was a longer run and the meter tails ran in metal trunking , or in a wall containing pipe work and there was a potential for it to happen, would this then warrant a code?

 

[loz2754]

Yes I understand, so if in a larger installation where there was a potential for this to happen for whatever reason , say between the head and the CCU was a longer run and the meter tails ran in metal trunking , or in a wall containing pipe work and there was a potential for it to happen, would this then warrant a code?

Of course, just like if the consumer unit is metal, not plastic.
And yes it would be potentially dangerous so a C2 would be appropriate.

And in the situation you describe, the long tails in metal containment, with a risk of a fault to earth happening, would have to have the 100mA s type RCD protection upstream.

 

[Storm93]

Of course, just like if the consumer unit is metal, not plastic.
And yes it would be potentially dangerous so a C2 would be appropriate.

And in the situation you describe, the long tails in metal containment, with a risk of a fault to earth happening, would have to have the 100mA s type RCD protection upstream.

Thanks for your help I appreciate it, I’ll take then there is no set regulation that says a TT system has to have a 100ma RCD main switch , however it is your engineering judgement at the time to determine the risk of fault conditions occurring and if the RCD 100ma is required!

 

[timhoward]

Thanks for your help I appreciate it, I’ll take then there is no set regulation that says a TT system has to have a 100ma RCD main switch , however it is your engineering judgement at the time to determine the risk of fault conditions occurring and if the RCD 100ma is required!

Maybe more helpful to keep in mind that a TT system requires RCD protection for fault protection, as the high Zs won’t allow enough fault current to operate breakers or blow a main fuse within the required disconnection time.
Exactly what form this takes can vary a bit. A fully loaded RCBO board achieves this.
Older installs might have say a 5 way board with an RCD main switch, or an up front RCD in its own enclosure.
The setup of the tails is indeed also a factor as to where it should go but it isn’t very common for this to influence things.

 

[timhoward]

Of course, just like if the consumer unit is metal, not plastic.

I might be misunderstanding your point.
Lots of properties near me are TT with double insulated tails going into a plastic CU with an RCD main switch.
Come the day these are upgraded to an new and obviously metal CU, I thought we’d arrived at it being ok to use a tails gland and tails clamp and bring the tails straight in?
A while back there was a debate that an up front RCD would need metal containment in 18th and the problem simply moved.
Or have I misremembered the conclusion from the pages of debate?!

 

[westward10]

BS7671 does not specifically state an rcd must be used for fault protection in a TT system although it is preferred. An overcurrent protective device may be used alone provided a suitably low value of Zs is permanently and reliably assured, see Regulation 411.5.2.
In an era when BS3036 semi-enclosed fuses were the norm then a 5A fuse in todays standards has a maximum Zs of 9.10 ohm.

 

[Strima]

I thought we’d arrived at it being ok to use a tails gland and tails clamp and bring the tails straight in?

As far as I'm aware this is acceptable. It best be as it's what I do... ?

 

[loz2754]

I might be misunderstanding your point.
Lots of properties near me are TT with double insulated tails going into a plastic CU with an RCD main switch.
Come the day these are upgraded to an new and obviously metal CU, I thought we’d arrived at it being ok to use a tails gland and tails clamp and bring the tails straight in?
A while back there was a debate that an up front RCD would need metal containment in 18th and the problem simply moved.
Or have I misremembered the conclusion from the pages of debate?!

I think what I meant to say is that you would have to take extra precautions to mitigate the possibility of a live to earth fault occurring within the consumer unit, if the consumer unit is metal, and the tails are not RCD protected. As opposed to not worrying about it with a plastic consumer unit. My apologies for not typing out what was in my brain!

 

[westward10]

Pre metal consumer unit it was a big no no to fit a metal enclosure consumer unit without rcd protection to its supply on a TT but now it seems okay, the times a changing.

 

[Strima]

Pre metal consumer unit it was a big no no to fit a metal enclosure consumer unit without rcd protection to its supply on a TT but now it seems okay, the times a changing.

But how many times did people use tails clamps/glands etc to mitigate the risk?