Reliability of measured Ze for Zs calculations

[Smugley]

Afternoon all.

Re-doing my calculations for a hot tub supply after client upgraded the specs.

Details are: Hot tub rated at 16A. Supplier advises to use Type C breaker to handle inrush when pump starts. 26m from CU to Hot tub via rotary isolator switch.

I was proposing to use a 20A Type C RCBO at the board, and take 4mm 3-core SWA from the CU all the way to the isolator switch. Run will be under 1st floor floorboards, through external wall, clipped direct to brick wall for 1m, then the rest underground. I chose to take the SWA all the way to the CU in order to keep the Zs down as low as possible. The calculated R1+R2 for 26m is 0.23 Ohms. (Not taking the armour into account). This would be fine at the moment as I measured the Ze as 0.15 Ohms, giving a total Zs of 0.38 Ohms. Plenty under the maximum allowable 0.93 Ohms for a 20A Type C RCBO.

Those who haven't fallen asleep yet will be asking Ah, but what type of supply is it?

Unfortunately it's a TN-S supply. So although unlikely, the Ze could rise up to 0.8 Ohms and the supplier would still be within the allowable limit. This would bring the design Zs up to 1.18 Ohms and in reality perhaps a bit higher. Should I be calculating the Zs based on the supplier's maximum allowable Ze? If so I'm going to have to install a huge cable to run a relatively small load. Is this overkill and do you think my choice of 4mm cable is acceptable?

Cheers

 

[ezzzekiel]

if you have read Ze at 0.38 then there is no problem it wont change unless there is a fault on the distribution side and if that was the case it wouldnt matter what size cable you had used.

 

[Des 56]

if you have read Ze at 0.38 then there is no problem it wont change unless there is a fault on the distribution side and if that was the case it wouldnt matter what size cable you had used.

Agree with ezzzekiel.go with what you've got

 

[Smugley]

Hi Ezzzekiel, yeah, the Ze reading was actually 0.15. Just wondering as technically speaking, I thought I should be basing a design Zs on the maximum permissable Ze rather than the measured value.

 

[Pinto Baz]

You can use your measured value, there is no reason to anticipate a change. If you are worried about disconnection time, the RCD part of your RCBO will cover you upto several hundred ohms (1666 I think)

 

[Smugley]

Thanks guys, pretty much what I wanted to hear! Never really came up before as I don't normally have a need for Type C devices.

 

[ezzzekiel]

would only ever use the max Ze if there was no other way of getting a reading - and even then it would depend on what final zs values wer going to be....

 

[nickblake]

well here one to think about , if your Zs does rise above the permited level set out by BS7671 , and its protected by and RCD ,then theres no problem as installing an RCD is one of the options for dealing with a High Zs , i know its better to get it within the levels of bs7671 but as that option is also in BS7671 then whats the problem

 

[Smugley]

Thanks for the comments. Typical of me, worrying unnecessarily

 

[poheZ]

yes the short answer to your question is no!