UK SPD in Existing metal clad CU?

[Initforthemoney]

Hi all!

Does an SPD have to go at the front end next to the main switch or can it be fitted with its MCB at the end of the row of MCBS on an existing CU to the far left?

EG main switch on the right, end of MCB row on the left will have the OCPD for the SPD, and then the SPD sat to the left of that?

Thanks for any pointers!

 

[pc1966]

It is better at the switch end since you have less inductance in the overall loop (for the high frequencies of a short lightning or load-switching surge the inductive impedance is normally far more important then resistance).

However, if the overall loop from L along busbar, through MCB to to SPD, back to N or E is below 0.5m (certainly below 1m) then no real problem. Using a C or D curve MCB helps as less inductance added in the "instant" trip coil.

See reg 534.4.8

 

[loz2754]

Lewden consumer units are designed in the way you describe.

 

[Initforthemoney]

It is better at the switch end since you have less inductance in the overall loop (for the high frequencies of a short lightning or load-switching surge the inductive impedance is normally far more important then resistance).

However, if the overall loop from L along busbar, through MCB to to SPD, back to N or E is below 0.5m (certainly below 1m) then no real problem. Using a C or D curve MCB helps as less inductance added in the "instant" trip coil.

See reg 534.4.8

Thanks.

Yes i was hoping to install the SPD MCB on the far left at the end of the existing MCBs with the SPD to the left next to the MCB so the only additional loop will be the busbar.

Lewden consumer units are designed in the way you describe.

It is indeed a Lewden unit!


Also, what is the consensus on SPD's when extending a lighting circuit? (Different job).

I did have a browse through a couple of other threads but there didn't seem to be any conclusion/solution.
This is in a domestic setting, with no new circuits involved.

Thanks.

 

[Pretty Mouth]

Also, what is the consensus on SPD's when extending a lighting circuit? (Different job).

I personally wouldn't bother fitting one just for this.

 

[pc1966]

Also, what is the consensus on SPD's when extending a lighting circuit? (Different job).

I guess it depends:

• If they have ever had issues of damage due to lightning then definitely.
• If in higher risk area and fed from overhead lines, probably yes as well (if you have a copy of the original 18th regs it has a lightning density map to help guide you but that aspect was dropped in AM2 due to complexity of applying in practice).
• If no special risk of a surge, and no specially high value of equipment that might be damaged, and just an extra light, etc, then probably not. But you can always ask the customer.

 

[Initforthemoney]

I personally wouldn't bother fitting one just for this.

I guess it depends:

• If they have ever had issues of damage due to lightning then definitely.
• If in higher risk area and fed from overhead lines, probably yes as well (if you have a copy of the original 18th regs it has a lightning density map to help guide you but that aspect was dropped in AM2 due to complexity of applying in practice).
• If no special risk of a surge, and no specially high value of equipment that might be damaged, and just an extra light, etc, then probably not. But you can always ask the customer.

Thanks.