Temporary supply TT earthing

[newfutile]

I have added a consumer unit and an earth stake to this so called temporary supply.
This is a full rcbo board apart from the mcb to the spd.

Would this need a upfront 300 ma or time delay rcd?

As its a metal case I have a proper tails gland
Ze=15.25 ohms with a earth fault current of almost 16 amps..

 

[timhoward]

Short answer, it's fine.

If you have the on site guide, see page 20, Figure 2.1(iii)
See also 2.2.6, in particular the last sentence of the NOTE on page 23

(The SPD wont' function properly if RCD protected so don't worry about that MCB and SPD inside the CU)

 

[newfutile]

Thanks for that , im just about to clip the tails to stop movement, i take it when our so called building works have finished I can use the supplied earth?
will the spd work with only 16 amp earth fault current at 230 volts anyway.
or maybe it will work better at clamping a surge ?

 

[timhoward]

will the spd work with only 16 amp earth fault current at 230 volts anyway.
or maybe it will work better at clamping a surge ?

Is it a 2 module one, with an L-N connection and a N-E connection?

 

[happysteve]

Regarding your question: make a judgement of the risk of the L conductor between the MCB and SPD falling out of the SPD and touching the metal case of the DB. If there is no realistic risk of this, due to the nature of how it's installed, then you're good.

You might want to put the TN-C-S earth bar in an enclosure (or replace with a Henley block), so you don't fall foul of 411.3.1.1 ("Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system...").

Looks very neat, btw!

 

[newfutile]

It is a 2 module SPD and it does not have its own mcb (my mistake) .

 

[davesparks]

The biggest issue I can see there is exposed terminals connected to 2 different earthing systems in close proximity.

 

[newfutile]

Yes they have a 22 ohms or so resistance between them, maybe fit a box over the DNO terminal?

 

[newfutile]

Some updates, I have secured the DNO earth and clipped the tails.
I did wonder how common it is for a dno to supply an earth from an overhead pole supply

 

[Rockingit]

As it's a PME, I'd just connect the DNO earth up to the MET along with your rod. All you're doing is increasing the 'M' (Multiple) part of the DNO's network. The reason that TNCS isn't allowed on building sites is because of risk of damage to the PEN during heavy construction phases - hence having a rod to 'guarantee'(?!) ADS in the event of a fault.

BS7430 says.. 4.2.3.4 "The TN-C-S system is also known as a 'protective multiple earthed' (PME) system. The PEN conductor, which is referred to as a combined neutral and earth (CNE) conductor, is earthed at the source and extremities of the distribution mains and points in-between hence the reference to multiple earthing. Multiple earthing of the CNE conductor ensures that if the conductor becomes open circuit for any reason, exposed-conductive parts remain connected to earth;...."

So what is happening here in practice is that your rod / TT is making a circuit back to the DNO's nearest earthing point (probably a tape under a pole), along with a gazillion other multiple paths, using the mass of actual planet earth to do so. This 'secures' a return circuit for safety / ADS in the event that the CNE isn't there due to damage (and logically, if the live/s aren't there either in the event of a completely severed cable then it basically doesn't matter....) - this is why you can't use a CNE on a building site, in case it get's damaged. But you can absolutely still use it when backed up with a temporary TT system.

 

[Lucien Nunes]

this is why you can't use a CNE on a building site, in case it get's damaged. But you can absolutely still use it when backed up with a temporary TT system.

Leaving all the various regs e.g. BS7375 aside for the moment, this doesn't follow logically IMO. Over a large PME system, the many local earth electrodes back each other up and provide a distributed low-impedance connection from the CNE to real earth. But if failure of the incoming CNE to the site itself is a specific elevated risk, none of the other electrodes are available as a backup; only the local rod will be connected to the MET and all the site's neutral current will be diverted into it which could easily result in a high touch voltage. This would not occur with a purely TT system, where severing an incoming neutral conductor has no impact on the touch voltage and merely stops the loads working.

What the regs make more of a point of, is the near impossibility of bonding all extraneous metal on a site to the CNE. Thus the equipotential zone is reliant on conductivity via true earth and therefore the MET should be as close as possible to true earth, achieved by TT rather than TN-C-S even with a parallel local rod, because the Ra will be much higher than the Zs and the MET potential determined by the prevailing external CNE conditions.

AFAIK neither BS7671 nor BS7535 explicitly prohibit TN-C-S but draw attention to the fact that it probably isn't feasible. E.g. 704.411.3.1 invokes the requirement for 411.3.1.2 which is the general use of EB. Then G12 from the ENA states that as this isn't usually practical, the TN-C-S earth shouldn't be provided on a site temp. Etc.

In summary, IMO the advantages of being completely disconnected from the TN-C-S earth and network conditions outweigh the disadvantages of relying on RCDs for fault protection due to the high Ra.

 

[Rockingit]

Leaving all the various regs e.g. BS7375 aside for the moment, this doesn't follow logically IMO. Over a large PME system, the many local earth electrodes back each other up and provide a distributed low-impedance connection from the CNE to real earth. But if failure of the incoming CNE to the site itself is a specific elevated risk, none of the other electrodes are available as a backup; only the local rod will be connected to the MET and all the site's neutral current will be diverted into it which could easily result in a high touch voltage. This would not occur with a purely TT system, where severing an incoming neutral conductor has no impact on the touch voltage and merely stops the loads working.

What the regs make more of a point of, is the near impossibility of bonding all extraneous metal on a site to the CNE. Thus the equipotential zone is reliant on conductivity via true earth and therefore the MET should be as close as possible to true earth, achieved by TT rather than TN-C-S even with a parallel local rod, because the Ra will be much higher than the Zs and the external system potential determined by the prevailing system CNE voltage.

AFAIK neither BS7671 nor 7535 explicitly prohibit TN-C-S but draw attention to the fact that it probably isn't feasible. E.g. 704.411.3.1 invokes the requirement for 411.3.1.2 which is the general use of EB. Then G12 from the ENA states that as this isn't usually practical, the TN-C-S earth shouldn't be provided on a site temp. Etc.

In summary, IMO the advantages of being completely disconnected from the TN-C-S earth and network conditions outweigh the disadvantages of relying on RCDs for fault protection due to the high Ra.

Totally see your logic. I guess it boils down to where we consider the potential break in the CNE to be, how many steps upstream. It follows that if our CNE breaks between actual earth points A - B then we have a 100% failure, but if we have a scenario of A,B,C,D...Z and it breaks at D then we've still got a web of A-D working co-operatively, parallel pathing to the remaining E-Z. I guess there's also a big distinction between 'construction' and CONSTRUCTION - where a simple new domestic house build (which is where I was coming from) is unlikely to have the issues of 'whole site current' and extraneous metal, whereas building a factory will have.