Zs reading does not meet current regs

[Robbo]

Doing some prior tests befor installing a fused spur from a radial circuit, and have found the Zs reading higher then the current regs, obviously it complys with a past reg, but as I’ll be doing a minor works for the job, when filling out their cert would I just put it down as an observation in comments on current installation and grade it C3.

Cheers.

 

[Charlie_]

 

[Deleted member 72666]

Deleted

 

[Deleted member 72666]

Yes it’s a 30mA bs 61009 type C RCBO.

...There's your issue!
What amperage?

 

[Deleted member 9648]

Earth fault disconnection times are met by the RCD, no requirement to change to a type B unless you want to waste your's or your customers money.
411.4.4 note 2 applies and 411.4.5

 

[davesparks]

You need to change it to a B type

They COULD change it to a B type, they don't NEED to change it to a B type. The regulations will be satisfied by the residual current element of the RCBO.

 

[ChrisElectrical88]

No need to change anything, it complys.

Next job.

 

[Midwest]

Just though, could it be argued that the Zs you're interested in is that relating to the fuse in the FCU, which for a 13A BS1362 is 1.8ohms for a 0.4 second disconnection time..

Well I go to the bottom of our stairs. I got disagrees before, on saying the exact opposite of what GBDamo is saying and getting dumb for here.

 

[Loki]

Reading is 0.79 at the furthest point, like I said it’s the only socket on the circuit, and is about 10-15mtr run from its DB

Hi,

You say it's a radial & that it is the only socket on the circuit & that Zs is 0.79

Yes a 30mA RCBO max is 1667 ohms


Just out of interest what csa is used on the radial? because if it's in 2.5mm on a 32A RCBO then the OCPD is to high for the cable ie. highest is 27A on clipped direct for t&e.

ie. an A2 radial is 4mm on a 30/32 amp OCPD, Zs stated in BBB for type C is 0.68 ohms (4mm is what I'm assuming is used on your radial)

An A3 radial is in 2.5mm on a 20 amp OCPD, Zs stated in BBB for type C is 1.09 ohms
So a 20A type C RCBO Zs at 80% is 0.872 ohms

 

[GBDamo]

Well I go to the bottom of our stairs. I got disagrees before, on saying the exact opposite of what GBDamo is saying and getting dumb for here.

That's why I like this game, and this forum.

You have to think as often there are many answers that will work, some unnecessary solutions and sometimes it's fine as it is.

 

[Deleted member 72666]

Well I go to the bottom of our stairs. I got disagrees before, on saying the exact opposite of what GBDamo is saying and getting dumb for here.

That's a Yorkshire saying if I ever heard one! Anyway, I thought you lived in a bungalow!

 

[GBDamo]

What about fault protection up to the spur.

Fair point, I had it in my head that spur origin was back to back, or side by side with the FCU in the same CSA cable as the original circuit. Then the new run to a DP isolator for the stair lift. In this I wouldn't be losing a great deal of sleep.

However it could equally be spurred, 10m cable run with FCU at the end, in which case you're right.

Any case it's on an RCBO so who cares, apparently.

 

[Deleted member 72666]

Earth fault disconnection times are met by the RCD, no requirement to change to a type B unless you want to waste your's or your customers money.
411.4.4 note 2 applies and 411.4.5

Ooh, you're so 17th Alan!

 

[Strima]

Using 1667 as a get out for poor design doesn’t sit right with me.

As asked before what rating is the RCBO, nearest C type is 32 amp at 0.68. What CSA is the cable and what is Ze and earthing arrangement?

 

[Nigel]

In this case then due to bad design/change in Regs the high Zs is through no fault of yours.

In this case I would utilise the RCBO for fault protection although. This is not permitted in Bs7671 so will need to be recorded on the MWC as a deviation.

 

[Nigel]

Changing it to a B Type may import nuisance tripping when the stairlift is starting.

Only a possibility but personally based on the info we have got I would leave it as a C Type and utilise the RCD for fault protection and note the deviation on the cert.