Voltage drop and ZS - Some thoughts

[pencilpusher]

Hi all

The scenario: Garden power supply to be taken from a 13am FCU spurred from a RFC. It then goes off to a double socket and lighting etc. No grief there. (RCD protection on RFC BTW!)

The spur is protected by a BS1363 13amp plug top fuse. Which ZS max do we use? Is it the ZS from the 13 amp fuse or from the origin, a 32 amp type B CB. My take is that it is the 13 amp fuse ZS.

Secondly, is voltage drop measured from the origin. ie around the RFC to the spur and then onto the garden lighting and power etc? or from the spurred position only.

Just a couple of things bugging me.

Thanks PP

 

[telectrix]

IMO both from the MCB in the CU. because the spur from the RFC is a part of the RFC circuit. it is not a circuit in it's own right.

 

[pencilpusher]

Thanks Tel, blimey you are quick!

 

[malcolmsanford]

When considering fault protection I would hope that your MCB at the board will disconnect faster then the BS 1363 Fuse. Though both should do it within 0.4 seconds for a final circuit on a TN system, in reality a MCB will do it in 0.1. In this case where I would spur of a RFC I also took both Zs and put them both on the EIC/MIEWC.

But to be honest as we are talking sockets and therefore the use of an RCD I would only really be putting on the Cert 1667

Technically yes VD should be taken from origin. So you take it from A-B CU to RFC Socket and then B-C RFC socket to Light, because the light will be 3%

 

[telectrix]

must be a 1st. only just come on a few minutes ago. lazy morning so far.

 

[topquark]

Can you guarantee that the fuse will give you the disconnection time you'd need (no) and as you're relying on the RCD for protection for equipment in the garden you need the "cooperation" of the RCD, so I'd say right back to the MCB/RCD.

 

[telectrix]

your MCB at the board will disconnect faster then the BS 1363 Fuse.​

​

always assuming that the 1363 is a fuse and not silver paper courtesy of messrs benson and hedges, or a bit of 1.5mm.

 

[pencilpusher]

Thanks guys, very helpful.

 

[pencilpusher]

your MCB at the board will disconnect faster then the BS 1363 Fuse.​

​

always assuming that the 1363 is a fuse and not silver paper courtesy of messrs benson and hedges, or a bit of 1.5mm.

I hope you are not suggesting I would do that ?! :53:

 

[brman]

I'd have said max Zs for the 1363 fuse - it is after all the protective device for that bit of the circuit.
If the Zs is too high to trip the MCB in time I don't see a problem as long as it trips the 1363 in time.

 

[topquark]

Thanks Tel, blimey you are quick!

He operates in a dimension outside of our space/time. He's even been known to answer before the question has been posted on occasion!

 

[pencilpusher]

I'd have said max Zs for the 1363 fuse - it is after all the protective device for that bit of the circuit.
If the Zs is too high to trip the MCB in time I don't see a problem as long as it trips the 1363 in time.

I had a feeling this may start an interesting debate!

 

[malcolmsanford]

The new sockets in the Shed will need to have RCD protection, so though you can achieve that with an RCD socket itself, 99 times out of 100 the RCD will be placed on the circuit itself.

If that is the case then the ZS will be 1667

 

[pencilpusher]

The new sockets in the Shed will need to have RCD protection, so though you can achieve that with an RCD socket itself, 99 times out of 100 the RCD will be placed on the circuit itself.

If that is the case then the ZS will be 1667

Yes. The garden sockets will be protected by the RFC RCD at source.

 

[malcolmsanford]

So that is the end of that debate