What would you put on the cert?

[Dave 85]

Did a job today where I had a 6mm T&E on a 32amp retrofit MCB (re used old circuit) going into a RCD spur (13amp) then a 2.5mm going off to a shower pump. There is also an unswitched 3amp fused spur halfway down the 2.5mm with a 1mm going to a mirror light.

Sounds a bit of a muddle I know but was the only way I could do it. 3036 Fuseboard full and hard to get cable into, existing lighting circuit impossible to extend as it was buried in concrete ceiling.

My question is, on the cert, in the overcurrent protection box do I state the 32amp 60898, 13 amp 1361 or 3 amp 1361. And in the cable box do I put the 6mm, 2.5mm or 1mm.
Im guessing I'd mark it down as a 32 amp 6mm circuit as the other bits are essentially fused spurs but just wondered if anyone thought different.

 

[Guest55]

No , your right - 6mm & 32A mcb.
The 32a mcb only protects the 6mm , spurs protect the 2.5 & 1.0
The device you record on the cert is always the one in the CU.

 

[telectrix]

agreed.^^^

 

[jdd]

Just to start a debate, surely the ocpd device protecting the point of utilisation would be the 1361's, so 2 minor works? otherwise the cert details won't make sense.

 

[Guest55]

Just to start a debate, surely the ocpd device protecting the point of utilisation would be the 1361's, so 2 minor works? otherwise the cert details won't make sense.

Thats actually not a bad question.

Its because the MWC and EIC ask for the CU device to be recorded , at the beginning of the circuit , not any local protection downstream.

 

[sedgy34]

going to be alteration to a circuit 6mm 32A

 

[Octopus]

So what happens when the Zs (taken at the extreme end of the circuit) exceeds the rating of the MCB but not the 13A fuse?

 

[malcolmsanford]

Well I'm hoping as this new cable would appear to be for a bathroom circuit that the Zs will be fine at 1667

 

[Plonker 3]

Think of it this way, if you install Switch Fuse Connection Units above a worktop for isolation of sockets underneath said worktop on a circuit what fuse do you put for OCPD?

 

[Guest55]

So what happens when the Zs (taken at the extreme end of the circuit) exceeds the rating of the MCB but not the 13A fuse?

Then your circuit design doesnt comply , quite simply.

 

[HandySparks]

It's an interesting question. I would say that, for the purposes of assessing whether the Zs is suitable for the overcurrent protective device, a fused spur is actually a separate circuit.

The definition of "Circuit" in BS7671 is: An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s). Clearly, for example, a 1mm² lighting spur supplied from a 2.5mm² ring via a FCU isn't relying on the 32A MCB in the board as its OPD; it is the BS1362 3A fuse in the FCU.

To test and certify the job properly, you may need to show separate test results for both the fused spur and the circuit it is fed from. The minor works form (in BS7671) doesn't refer to a DB, only "Protective device for the modified circuit", which could reasonably be a BS1362 fuse. The "generic schedule of test results" does refer to "Zs at DB", but maybe they didn't have enough space to add "or other circuit origin". As I think someone else said, think of the FCU as a one way DB. Clearly the schedule is laid out to cover the general case rather than the particular one of a FCU as the circuit origin, but that shouldn't stop us thinking it through properly.

In practice, I guess most of us just test and certify short spurs as part of the main circuit, but there are some cases where it makes sense to do them separately.

 

[malcolmsanford]

Good argument there but Zs is nothing to do with overcurrent

 

[HandySparks]

I see where you're coming from Malcolm, but an overcurrent may be an overload current or a fault current (note at the bottom of 430.3). Of course, Zs is about how quickly an earth fault current will cause the OPD to open.

 

[Guest55]

It's an interesting question. I would say that, for the purposes of assessing whether the Zs is suitable for the overcurrent protective device, a fused spur is actually a separate circuit.

The definition of "Circuit" in BS7671 is: An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s). Clearly, for example, a 1mm² lighting spur supplied from a 2.5mm² ring via a FCU isn't relying on the 32A MCB in the board as its OPD; it is the BS1362 3A fuse in the FCU.

To test and certify the job properly, you may need to show separate test results for both the fused spur and the circuit it is fed from. The minor works form (in BS7671) doesn't refer to a DB, only "Protective device for the modified circuit", which could reasonably be a BS1362 fuse. Thats a big leap in interpretation you must admit. Putting details on the cert of only the spur means there is no indication of which CU circuit was worked on.

The "generic schedule of test results" does refer to "Zs at DB", but maybe they didn't have enough space to add "or other circuit origin". They dont need to , the CU is the origin of any final circuit.


As I think someone else said, think of the FCU as a one way DB. No its not , they have completely different BS numbers that will be referenced in the regs under definitions.

Clearly the schedule is laid out to cover the general case rather than the particular one of a FCU as the circuit origin, but that shouldn't stop us thinking it through properly.

In practice, I guess most of us just test and certify short spurs as part of the main circuit, but there are some cases where it makes sense to do them separately.

Some good points raised though.
But to follow your example to its conclusion , your implying that the cable from the CU to the spur is a sub-mains if the spur is a DB , and the standard circuit arrangements shown in the regs do not back this up.

 

[HandySparks]

Some good points raised though.
But to follow your example to its conclusion , your implying that the cable from the CU to the spur is a sub-mains if the spur is a DB , and the standard circuit arrangements shown in the regs do not back this up.

Not a "sub-main" as such, just that it's possible to have a circuit supplied by another circuit, even if you'd describe both as "final circuits"!

I think that the people who write the regs tend to think about "standard" arrangements rather than the more untidy reality that we deal with every day.