Max Zs exceeded from generator

[Caerdydd]

Hypothetical situation, power supplied from a generator - So a temporary installation.

BS 61009 Type C 100mA RCBO – 32A fitted at generator

Cable run is approx. 300m, in 4mm HO7 Rubber PVC flexible cable ran outdoors, terminated in a 32A socket (single phase)

Ze = 0.03 ohm
Maximum Zs is 0.72 ohm

Calculated Zs is 1.89 (0.03 + 1.86)

If a distribution box is fitted at the end of a cable run, with 4 x 16A sockets wired so there are 2 16A Type C RCBO's, each feeding 2 x 16A Sockets would this offer enough protection as the Zs has been exceeded.

Load on the cable is approx 6.5A, so does exceed the max voltage drop.

I know this isn't ideal, and the cable run should either be shorter, or wired in a bigger size cable. But it's a weird situation.

 

[spark 68]

Max Zs (assuming 30mA RCBO) is 1667 ohms.

Edit: just noticed 100mA RCBO max Zs is 500 ohms

For temporary supplies all socket outlets up to 32A and mobile equipment supplied with flexible cables shall be protected by a 30mA RCD

 

[Caerdydd]

Thanks, RCD at generator protected at 100mA, to provide discrimination from 30mA RCD at end of cable.

 

[Guest123]

Thanks, RCD at generator protected at 100mA, to provide discrimination from 30mA RCD at end of cable.

Unless it's T/D, it wont.

 

[spark 68]

In section 740 (BGB), it states all final circuits (including the ones I listed earlier) shall be protected by a 30mA RCD.

i) lighting
ii) socket outlets rated up to 32A, and
iii) mobile equipment connected by flexible cable with a CCC up to 32A

A note is given underneath: The requirement for additional protection relates to the increased risk of damage to cables.

This to me would suggest that the 30mA RCD/RCBO should be before the flexible cable.

furthermore this requirement does not apply to:

i) circuits protected by SELV or PELV, or
ii) circuits protected by electrical seperation, or
iii) lighting circuits placed out of arms reach blah blah .

So as the OP's circuit is supplied by a small generator mounted on a truck, would this be classed as electrical seperation ?, I know he has installed a rod at the generator (Added to existing arrangement), and he seemed to think it was TN-S as originally the earth is connected to the chassis of the generator and/or truck with no rod as an earth reference.

If the generator did not have the Earth rod fitted would it then be classed as a 'floating' electrically seperated supply ?

My thinking here is that by adding the earth rod has the OP made the requirements more onerous than they need have been.

I haven't had that much experience with this type of install.

 

[spark 68]

I think I have found the answer in reg section 551.4.4

Additional requirements for protection by ADS where the installation and generating set are not permanently fixed, it seems all circuits require 30mA RCD protection (551.4.4.2), regardless of whether the earth rod is present or not.

 

[Guest123]

@ OP, unsure exactly what you are getting at.

There are many things to consider when using a genny for a public supply.

I've attached the HSE guidance on it plus a link to the IET Wiring Matters issue containing info on genny supplies.

Wiring Matters - 2011 issues - IET Electrical


Issue 39, summer 2011.

 

[Rockingit]

Hmmm....generally speaking you would consider (in practice) the first cable from genset to distro box as a submain, which means that your 'final' or sub-sub feeds are coming via the 30mA RCD, so you're good to go with the 1667Ohms scenario. All the upfront RCBO is doing is really offering the genset some protection from overload (with a nod towards protecting someone if they manage to nadge the flex). Many larger sets that I work with have key-switch enable for the upfront RCD's in the set, as with very large distribution networks even time-delay RCD's can cause problems. Also, it's worthwhile remembering that because you have been a good sparky and banged in a nice big earth rod right by the set, it's a TT system anyway if you want it to be.