Looking for opinion on whether this scenario is compliant

[magnoliafan89]

I tested a tt system yestersay and the zs at db was 9.11 ohms with ipf of 25a, on the socket circuit i got a zs of 10 ohms and an r1+r2 reading of 0.49 ohms and was told this was fine....i always thought and was taught a tt would give higher readings for a zs than as given in appendix table b6. Now im confused

 

[Bellendian]

The higher zs is fine, and dealt with by the application of rcds for fault protection, and in this case since they are 30ma they will also provide additional protection. The curve ball is the 2.64 ohm L-N....generally we assume that if R1+R2 is good then R1+RN would be as well....since the CSA of cpc will be less than or equal to the neutral...but ring finals aside, what specific tests are mandated to check this? Could well be a loose neutral connection, but how are these tested?
Often hear circumstantial evidence that it's always neutrals that burn out or are loose....is it because a loose l or cpc gets picked up during r1+r2 testing, but those pesky neutrals fall by the wayside? Just a thought....

 

[Wilko]

Doesn't need to comply across L/N as no values are given for this scenario. We need to know the L/N reading at source.

A different time to comply perhaps?
Reg 434.5.2 says to interrupt fault current before damage has occurred. So we don't need 200ms but we do need to check dis times with adiabatic (?).

 

[Bellendian]

A different time to comply perhaps?
Reg 434.5.2 says to interrupt fault current before damage has occurred. So we don't need 200ms but we do need to check dis times with adiabatic (?).

Agreed, but this is design stuff, and possibly something that is missed during initial verification? Or am I losing the plot!?

 

[Strima]

Compliance should be checked each time the installation is inspected and tested as supply nature and perameters change.

A circuit that complied when designed may not comply now.

 

[Bellendian]

Yep, but on a typical radial cct, which of the mandated tests would highlight a higher than expected neutral impedance?

 

[johnduffell]

A different time to comply perhaps?
Reg 434.5.2 says to interrupt fault current before damage has occurred. So we don't need 200ms but we do need to check dis times with adiabatic (?).

Good point although adiabatic should only be used up to 5s, go far beyond that and the value would be way too conservative.
Not sure how to work it out, i agree with the rest of your post. I feel like we should be sizing it as an overload protection, so as long as the mcb can protect the cable that way it's ok? There's no special case of worrying about exposed conductive parts being live in this case.

 

[Wilko]

If there isn't a defect in the DNO supply or the RFC, then the RFC must be very long to get L-N of 2.6 Ohms at an outlet. Either way, the voltage drop would slow down my tea and toast, no good at all !

 

[The Ghost]

The rcds are providing fault protection

Are they? This is a dual RCD board and if I understand correctly the RCD are additional protection. As this is a TT system why would one not use an up front 100ma RCD for fault protection. I must admit I very rarely if ever encounter TT and if I do usually get it converted to PME so frankly I am not on top of the implications reg wise of TT.

 

[Ian1981]

Are they? This is a dual RCD board and if I understand correctly the RCD are additional protection. As this is a TT system why would one not use an up front 100ma RCD for fault protection. I must admit I very rarely if ever encounter TT and if I do usually get it converted to PME so frankly I am not on top of the implications reg wise of TT.

30mA rcds offer fault protection for earth faults only and have the added bonus of offering additional protection hence the 2 tests.
1x rating trip within 200ms for circuits upto 32 amps if it’s TT (the standard for bs61008 and 61009 being 300ms) and 5x for additional protection within 40ms.
Your mcbs will be providing short circuit and overload protection only.
It maybe that additional protection via 30mA rcd is required for socket outlets,cables serving a bathroom location and cables buried in walls etc rather than relying on a 100mA rcd which would not comply with the requirements of additional protection and only offers fault protection.
I would not be worrying about your high Zs readings after all it’s a TT arrangement and as your covered for earth faults via the dual rcds in the consumer unit.
what your issue is with the bonding requirements will only be applicable if you have extraneous conductive parts entering the dwelling.

 

[Wilko]

Hi - agree.
But any L-E faults before the CU's RCDs will have to clear via the service fuse. And with TT its likely it won't clear and there's risk the installation E could rise. Case of L tail flapping about inside AMD3 CU for example (very unlikely and why tails glands and good workmanship always needed).

 

[Ian1981]

Hi - agree.
But any L-E faults before the CU's RCDs will have to clear via the service fuse. And with TT its likely it won't clear and there's risk the installation E could rise. Case of L tail flapping about inside AMD3 CU for example (very unlikely and why tails glands and good workmanship always needed).

I like to install an S type rcd rather than the main switch if using a dual rcd board but that’s just my preference on TT arrangements.

 

[westward10]

This is a plastic CU doesn't need a 100ma main switch. The rcds in situ provide additional and fault protection as already stated.

 

[Ian1981]

Yes agreed a plastic consumer unit takes the danger scenario of loose tails touching the enclosure causing a potential hazard, out of the equation

 

[Adam W]

Yes agreed a plastic consumer unit takes the danger scenario of loose tails touching the enclosure causing a potential hazard, out of the equation

... Although of course there's still the risk of the CU catching fire.
I had a quick leaf through the new OSG in the wholesalers the other day - there's bit in it about countering precisely that situation, but I can't remember the exact details.

 

[rompling]

The higher zs is fine, and dealt with by the application of rcds for fault protection, and in this case since they are 30ma they will also provide additional protection. The curve ball is the 2.64 ohm L-N....generally we assume that if R1+R2 is good then R1+RN would be as well....since the CSA of cpc will be less than or equal to the neutral...but ring finals aside, what specific tests are mandated to check this? Could well be a loose neutral connection, but how are these tested?
Often hear circumstantial evidence that it's always neutrals that burn out or are loose....is it because a loose l or cpc gets picked up during r1+r2 testing, but those pesky neutrals fall by the wayside? Just a thought....

Totally agree