High Zs on submains and high Ze

[rugbylad]

Hi chaps.

On some remedial works at a school, I have a sub main a 16mm armoured feeding it about 50m away from the main DB. Fed from a C63 triple pole 60898.
The issue I have is the Zs at this submain is 0.46 which is way over the maximum permitted.
But the Ze at the main DB is 0.35. On a tncs btw, which I feel maybe too high also.
How can we bring this down? Or can this be noted down on the test sheets that the Ze is high hence the high Zs reading?

Thanks

 

[Dillb]

Has this been highlighted before or after the works have been carried out?

 

[Dillb]

You cant use a high Ze as a reason that your Zs values dont comply.

 

[rugbylad]

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.

 

[Dillb]

What do you mean you have done everything to lower the values? Either installing a larger earth cable to the submain or swapping the OCPD will ensure it complies.

 

[rugbylad]

Already has the supplementary bonding, I was considering changing the mcb to a B63 but surely that will trip before the Ctype breakers in the submain DB? Will it not?

 

[Dillb]

Changing the design of the circuit is a bad idea. You need to do some cable calculations and find out what size earth cable it needs.

 

[Nigel]

What do you mean you have done everything to lower the values? Either installing a larger earth cable to the submain or swapping the OCPD will ensure it complies.

Not really going to work as the max permitted Zs for a C63 is higher than the Ze. You could put in 10 x 400mm earths it will still not be within limits.

 

[Rpa07]

As the Zs is high for a TNCS could the DNO be contacted for investigations prior to any calcs and changes. They may have a take on it and be able to sort from their upstream side.

 

[Nigel]

You cant use a high Ze as a reason that your Zs values dont comply.

Not strictly correct.

 

[rugbylad]

I think contacting the DNO is my best bet here eh?

 

[Dillb]

Not strictly correct.

The OP worded the question as though the works had been carried out, its now a lot clearer what has happened. Given that they say the max Ze of a TN-C-S can be 0.35 it has been badly designed from the start.

 

[Nigel]

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.

 

[Nigel]

The OP worded the question as though the works had been carried out, its now a lot clearer what has happened. Given that they say the max Ze of a TN-C-S can be 0.35 it has been badly designed from the start.

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

 

[rugbylad]

No, my OP states I’m doing remedial work

 

[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.