Max Zs on rcbo type c 32a 1667 or 0.54??

[SDSHAMMER]

Has anyone any views on max Zs for say a type c 32a rcbo at 80% is 0.54 and 100% 0.68. I am currently testing a collage and all ring circuits are type c and I am exceeding 0.54 and 0.68 this is a TNCS system so to my knowledge table 41.5 in regs which tells us 1667 for 32a rcbo for TT systems but I have guys telling me this is fine for TNCS as well. I would disagree as the installation hasn't been designed properly as there is no need for type c and all circuits are wired in 2.5mm pvc singles. If type c was really needed why did they not just use 4mm pvc singles to bring max Zs down? Thoughts Much appreciated guys. Think next step is for Nicec to make final call

 

[Leesparkykent]

Bad design Imo but compliant.

 

[Deleted member 26818]

it is acceptable to use an RCD to achieve ADS when the Zs is too high for other methods.
What is worrying, is whether the test schedule will be conducted, to ensure the RCDs will operate in the event of a fault.

 

[Pete999]

Has anyone any views on max Zs for say a type c 32a rcbo at 80% is 0.54 and 100% 0.68. I am currently testing a collage and all ring circuits are type c and I am exceeding 0.54 and 0.68 this is a TNCS system so to my knowledge table 41.5 in regs which tells us 1667 for 32a rcbo for TT systems but I have guys telling me this is fine for TNCS as well. I would disagree as the installation hasn't been designed properly as there is no need for type c and all circuits are wired in 2.5mm pvc singles. If type c was really needed why did they not just use 4mm pvc singles to bring max Zs down? Thoughts Much appreciated guys. Think next step is for Nicec to make final call

Why should NICEIC make the final call? they don't make the rules, they just take your money

 

[SDSHAMMER]

Yes spinlindon that is my main concern. The regs don't give a definitive answer but i myself would design a circuit to meet disconnection times in event of a fault current irrespective of the protection device. Yes pete999 they are good at taking our money, making their own rules and letting anyone join them IE Cowboys

 

[Simonslimline]

Bad design Imo but compliant.

Considering it is a TN-C-S system, i agree on bad design.

I wonder if it was always a TN-C-S, or could it of been converted from a TT possibly? Who knows.

 

[hightower]

I fail to see why the theory that 1667 ohms can apply to TT systems when they fail to meet disconnection times, but it is not suitable on TNS/TNCS systems. Maybe not the best design in the world, but surely what's good for the goose is also good for the gander?

However, if it's all in 2.5 singles (line, neutral, and earth) surely that means if Zs doesn't comply, the short circuit (line & neutral) will have the same resistance and thus it won't comply - the RCD will deal with the high resistance in the event of fault current, but surely there's a chance the breakers won't break (in time) in the event of short circuit current?

 

[telectrix]

I fail to see why the theory that 1667 ohms can apply to TT systems when they fail to meet disconnection times, but it is not suitable on TNS/TNCS systems. Maybe not the best design in the world, but surely what's good for the goose is also good for the gander?

However, if it's all in 2.5 singles (line, neutral, and earth) surely that means if Zs doesn't comply, the short circuit (line & neutral) will have the same resistance and thus it won't comply - the RCD will deal with the high resistance in the event of fault current, but surely there's a chance the breakers won't break (in time) in the event of short circuit current?

holy fuses, batman. good thinking for so early before beer time. :stuart:

 

[Simonslimline]

I fail to see why the theory that 1667 ohms can apply to TT systems when they fail to meet disconnection times, but it is not suitable on TNS/TNCS systems. Maybe not the best design in the world, but surely what's good for the goose is also good for the gander?

However, if it's all in 2.5 singles (line, neutral, and earth) surely that means if Zs doesn't comply, the short circuit (line & neutral) will have the same resistance and thus it won't comply - the RCD will deal with the high resistance in the event of fault current, but surely there's a chance the breakers won't break (in time) in the event of short circuit current?

So the next question is, are there required disconnection times for short circuit faults in BS7671! I am pretty sure it is just EFLI which it covers. 411.3.2.

 

[hightower]

So the next question is, are there required disconnection times for short circuit faults in BS7671! I am pretty sure it is just EFLI which it covers. 411.3.2.

But wouldn't 434.5.2 come in to play?

Just quickly done the maths on that. Can someone confirm my working for my own learning. Adiabatic says about 80 seconds to the limit, but the short circuit current will be over 400 amps which will break a type C in the required time still. So in this case it is fine and complies - can someone confirm I'm on the right tracks?

 

[hightower]

Just redone calculations as OP didn't give us the reading he was getting. Am I right in saying any reading on ze + r1+rn greater than 3 ohms won't achieve compliance with 434.5.2? I know 3 ohms is a substantial figure, and very likely won't be this high in OPS case - just trying to see if I'm approaching this the right way

 

[Risteard]

ze + r1+rn

Not Ze though as it isn't earth fault loop impedance you are considering.

 

[hightower]

Not Ze though as it isn't earth fault loop impedance you are considering.

Yeah sorry, I was searching for the terminology but struggled. Would it be called the 'loop' reading?

 

[Simonslimline]

Yeah sorry, I was searching for the terminology but struggled. Would it be called the 'loop' reading?

Z pn or Z ln i guess.

 

[Pete999]

ZZZZZZZZZZS lol sounds like my Mrs when She''s kippin