Question about Zs and PSCC needs some clarification thanks

[King84]

Hi everyone

Recently I have come across an advert of someone selling Afdds C type and the person mentioned the need to sell because Zs was too high so he was considering changing the circuit breaker back to B type as it allows enough Zs to meet the regulations.

As Afdds is also combination of Rcds so Zs would be 1667 ohms which someone mentioned to which he replied for earth fault loop yes but for L-N loop doesnt meet regulations hence the replacement.

Furthermore he says for L-N doesnt meet reg for short circuit thats why we do PSCC. If TNC-S arrangement then because of Neutral and earth are connected so we do not need to do PSCC and Prospect earth fault current would be same as short circuit.

As I am also in a learning stage so always mess up some concepts when I come across these questions so some questions emerged in my mind which if someone could answer and correct me if I am wrong .

My understanding was we carry out Ipf at origin to check if there is enough fault current generated if Supply side L-E comes in contact to disconnect Main cutout fuse or Circuit breakers hence we also consider short circuit capacity of MCBS.

By checking PSCC at origin we ensure that if L-N at supply side comes in contact it would disconnect main cutout fuse or MCBS.

on the load side afcourse as fault current depends upon the MCBs such as 32type B MCB would have minimum of 160A of fault current hence 230/160*0.95 would make 1.37 ohms resistance max to meet disconnection times.

Since the person mentioned he checked PSCC and it did not meet Zs so he is considering it back to type B, does he mean that, he carried out PSCC at supply side or towards load side on final circuits?
As on load side we usually carry out Zs to ensure it meets reg and since the Afdds contains RCD so it would meet regs.
Do you guys carry out PSCC on final circuits to see it circuit breakers would trip if short circuit were to happen like he did ? as my understanding was to only check Zs to meet regs and he said Zs was ok but PSCC was not met.
Straight after the post he took it off and I could not question him but the question is bugging me .

 

[timhoward]

There's quite a lot in that question!

It sounds like he measured or calculated a Zs, and found the result exceed the requirements to meet disconnection times. The type C max Zs values are a lot lower than type B, e.g. a C32 MCB has a max Zs of 0.68 (80% value is 0.54) which is a lot easier to exceed, especially if the Ze is on the high side.

(Just because an AFDD or RCBO contains RCD technology doesn't automatically make the max Zs 1667 ohms. An earlier decision regarding whether RCD's are being used for fault protection or not dictates that.)

Do we check PSCCs for final circuits? On TN systems PSCC will be equal or higher to PEFC as earth conductors are always equal or smaller in size compared to their respective line conductors. So if PEFC is sufficient to meet disconnection times, PSCC will be too.

 

[Lucien Nunes]

On TN systems PSCC will be equal or higher to PEFC as earth conductors are always equal or smaller in size compared to their respective line conductors.

Not necessarily, for MIMS or singles in metallic containment. For MICC, 4H1.5 is the extreme case with a 14sqmm sheath, where on a long circuit from a TN-C-S supply the PSCC will be little over half the PEFC.

 

[timhoward]

Not necessarily, for MIMS or singles in metallic containment. For MICC, 4H1.5 is the extreme case with a 14sqmm sheath, where on a long circuit from a TN-C-S supply the PSCC will be little over half the PEFC.

Thanks, and great point - my brain was unfortunately in domestic mode where such things are of course rare these days.

 

[King84]

Thanks alot for the responses .

so We check on Supply side of consumer unit for PSCC L-N to ensure if meter tails L-N accidently comes in contact, there is enough fault current generated to be disconnected by the main fuse?

On the load side if L-N comes in contact then Protected devices such as MCBs can clear that short circuit fault current.

I thought originally in the initial post by another fella on other forum meant that he conduct specific PSCC for supply side which we all do and write the highest of PEFC and PSCC but when he only advertising on the forum for his Afdds for 60quid and the reason he gave was L-N loop does not meet regulation requirements but L-E, N-E does so just because L-N loop does not hence he changing back from C32 to B32 and selling the C32AFDD this got me confused

 

[King84]

Do we check PSCCs for final circuits? On TN systems PSCC will be equal or higher to PEFC as earth conductors are always equal or smaller in size compared to their respective line conductors. So if PEFC is sufficient to meet disconnection times, PSCC will be too.

on TNC-S , as Terre and Neutral are combine hence PSCC and PEFC isnt the same as they are combined at one point?
and on TNS as they are separate so PEFC as earth is smaller in size hence during faulty scenario when Earth and L comes in contact shouldnt it generate less fault current than PSCC when Neutral would be same size as Line conductor ?

 

[timhoward]

the reason he gave was L-N loop does not meet regulation requirements but L-E, N-E does so just because L-N loop does not hence he changing back from C32 to B32 and selling the C32AFDD this got me confused

It could in fact be the very scenario that @Lucien Nunes mentioned, e.g. if steel containment is used as a CPC then the CPC component can have a very low resistance. So L-E could be within the permitted Zs (cable + steel containment), but L-N (2 cables) would have a higher resistance.

on TNC-S , as Terre and Neutral are combine hence PSCC and PEFC isnt the same as they are combined at one point?

Yes on TNCS one would indeed expect them to be the same.

on TNS as they are separate so PEFC as earth is smaller in size hence during faulty scenario when Earth and L comes in contact shouldnt it generate less fault current than PSCC when Neutral would be same size as Line conductor ?

Yes

 

[davesparks]

so We check on Supply side of consumer unit for PSCC L-N to ensure if meter tails L-N accidently comes in contact, there is enough fault current generated to be disconnected by the main fuse?

No, we need to know the PSCC to confirm that the protective device installed can withstand the maximum possible fault current. So if your protective devices have a maximum shirt circuit rating of 6kA you need to make sure that the PSCC doesn't exceed this.

You check the measured Zs to ensure that the protective device will operate within the required time.

 

[timhoward]

No, we need to know the PSCC to confirm that the protective device installed can withstand the maximum possible fault current. So if your protective devices have a maximum shirt circuit rating of 6kA you need to make sure that the PSCC doesn't exceed this.

Something I've never been 100% clear on....if the supplier fuse is a BS 1361 type 2 rated to 16Ka, in the scenario you mention is the provision of higher rated (e.g.10Ka) devices in the customer unit (which I'm assuming meets BSEN 60439-3)
a) good practise or
b) required
(I'm thinking more about commenting on existing installations that new installations)
Thanks

 

[Aaron b]

Something I've never been 100% clear on....if the supplier fuse is a BS 1361 type 2 rated to 16Ka, in the scenario you mention is the provision of higher rated (e.g.10Ka) devices in the customer unit (which I'm assuming meets BSEN 60439-3)
a) good practise or
b) required
(I'm thinking more about commenting on existing installations that new installations)
Thanks

It's the exception of 434.5.1 if I understand it correctly. I asked some questions on it a while back and it sounds like it's very unlikely to be an issue due to the resistances between the supply and the fault quickly adding up, lowering it to something more acceptable.

 

[pc1966]

It's the exception of 434.5.1 if I understand it correctly. I asked some questions on it a while back and it sounds like it's very unlikely to be an issue due to the resistances between the supply and the fault quickly adding up, lowering it to something more acceptable.

What you are looking at with 434.5.1 is the "cascading value" for protective devices.

Basically in the domestic case the upstream supply 60-100A fuse limits the peak fault current to a level that downstream domestic 6kA MCBs are able to break even when the measured PSCC is above 6kA.

Also in practice you rapidly lose PSCC as you move away from the CU on any final circuit. Even if you had an infinite PSCC at the supply, to bring it to 6kA needs 38 mOhm and you get that from around 1m of 1mm T&E or around 4m of 6mm T&E.

 

[pc1966]

See this guide:

https://www.beama.org.uk/static/uploaded/992ea5f3-2044-46b2-9db965480b99d7c8.pdf

 

[pc1966]

on TNC-S , as Terre and Neutral are combine hence PSCC and PEFC isnt the same as they are combined at one point?
and on TNS as they are separate so PEFC as earth is smaller in size hence during faulty scenario when Earth and L comes in contact shouldnt it generate less fault current than PSCC when Neutral would be same size as Line conductor ?

Mostly.
In the TN-C-S case you would expect the PSCC and the PFC to be the same as they are both using the same L and PEN impedances, with the tail N and main earth connection being short and usually of negligible overall effect.

The OSG has a max TN-C-S Ze for this as 0.35 ohm which for a nominal 230V would be 660A and that would give you about a 9% drop on 60A. That is, of course, more than the regs permit for circuit VD but the DNO have different rules to follow. But as a sanity check if you measure Ze above this on a TN-C-S setup then something is wrong. Same for seeing a significant difference in PSCC and PFC.

In the TN-S case the usual expectation is the supply CPC is smaller than the N and so PFC will be less than PSCC. But that is not always the case as you can have the earth-bond going to shared metal service pipes that are in turn bonded to other supply earths and so the PFC measured in-use (for Zs at DB) is higher than the PSCC since the parallel earths have lowered the CPC impedance to below that of the supply N cable.

However, you should always base the design on the supply Ze and not the in-use Zs at DB as in the future those metal service pipes may be replaced by plastic, etc. Usually you want to be able to disconnect on the OCPD (switch-fuse, MCB, etc) as it is simple and reliable, but if the Ze is too high to permit it than you have to look at using RCDs and then the selectivity of cascading them, etc.

 

[pc1966]

Just to add to the above discussion: the DNO fuse does not have to meed 5s disconnection on earth faults as the regs for a sub-main would dictate. Again, they operate under different rules and that fuse is to protect their network from overload-style faults, not to provide shock protection ADS for downstream use as the BS7671 wiring regs wiring dictate.

Just in case anyone asks about the often quoted TN-S max Ze of 0.8 ohms being too high for even a 60A fuse!

 

[Aaron b]

Just to add to the above discussion: the DNO fuse does not have to meed 5s disconnection on earth faults as the regs for a sub-main would dictate. Again, they operate under different rules and that fuse is to protect their network from overload-style faults, not to provide shock protection ADS for downstream use as the BS7671 wiring regs wiring dictate.

Just in case anyone asks about the often quoted TN-S max Ze of 0.8 ohms being too high for even a 60A fuse!

but would this then be treated like a TT with all conductors (but not busbars) needing to be double insulated until the first protective device, assuming the consumer unit was metal and an RCD was required for fault protection for outgoing circuits?