High Zs on submains and high Ze

[Dillb]

I agree it has been badly designed. That said the Ze may not have always been 0.35.

True, but it should always designed for the worse case scenario as the DNO will probably just say well its within our acceptable limits.

 

[rugbylad]

In this scenario you have two options.

1. To encase the complete cable run in material that will contain any fire risk that may occur from the OCPD not tripping in time.

2. To install RCD protection to the cable as RCDs have far higher maximum permitted Zs values.

The first option is not really an option due to cost so this only leaves the second option. A 500mA RCD has a max permitted Zs of 100 ohms so I would advise installing one of these at the supply end of the cable.

If you are doing an EICR I would code it as a C2 if there is currently no RCD protection on the cable.

Funnily enough it is in metal trunk all the way to the DB.
Think I’ll speak to my QS about the RCD option.

Many thanks for the advice.

 

[Nigel]

True, but it should always designed for the worse case scenario as the DNO will probably just say well its within our acceptable limits.

No. You would not design a TN-S installation based on a max Ze of 0.8.

 

[Nigel]

Funnily enough it is in metal trunk all the way to the DB.
Think I’ll speak to my QS about the RCD option.

Many thanks for the advice.

It would need to be isolated from other cables so in its own trunking. You would also need to consider down time of supply. Personally I would stay away from this option and install a 500mA RCD.

 

[Nigel]

Another option would be to change the OCPD from a 60898 to a BS88. BS88 would have a max permitted Zs of 0.66 ohms. Feeding DBs from DBs is never a good idea.

This is assuming there is access to a bus bar chamber though.

 

[rugbylad]

No busbar chamber unfortunately.

 

[westward10]

Does it need to be 63A.

 

[Nigel]

Yeah, it was picked up on an eicr we done last year. It’s just everything has been done to lower the values but to no prevail.

Out of interest what have you done to lower the values to lower than the Ze?

 

[rugbylad]

Out of interest what have you done to lower the values to lower than the Ze?

Just what I can reasonably do practically, tightened all connections, contact cleaner, supplementary bonding.

 

[rugbylad]

Does it need to be 63A.

Unfortunately yes.

 

[Nigel]

Just what I can reasonably do practically, tightened all connections, contact cleaner, supplementary bonding.

But there is nothing that can be done to get a Zs lower then the Ze so it was always a design issue.

 

[ChrisElectrical88]

Can you not put the Main Tails into Henley blocks and install a 60A Fused Switch as previously stated.

I understand some schools this will not be possible due to the cables coming from an outside substation into a panel board. However with your Ze of 0.35 I would assume the school is not large enough for its own transformer and sub.

 

[Ian1981]

S type rcd to protect the sub mains perhaps?
Assuming that you still will achieve short circuit protection with the C type mcb

 

[Wilko]

Hi - with B63 the max goes up from 0.35 to 0.69 and job's done (?).

 

[Nigel]

Hi - with B63 the max goes up from 0.35 to 0.69 and job's done (?).

I don't think a B type 60898 will be suitable for a distribution circuit.