Protecting Tails

[horsleya]

I am trying to establish what level of protection is require for the Tails between the service head and the CU.

I understand that these should be less then 3M long, 25mm CSA and double insulated.

As these tails are only protected by the 100A service head fuse I would have thought that additional earthed mechanical protection would also be require but see little mention of this.

Can anyone summerise what their understanding of the req's is?

Does a TT system require additional protection for the tails over a TN based system?
I am just referring to the tails as I understand the RCD requirements for the final circuits.

Many thanks

 

[Sintra]

A single RCD protecting a TT system must be installed at the origin of the installation unless the part of the system between the origin and the RCD fulfills the requirements for protection by Class II equipment or equivalent insulation. 531.4.1

As long as your supply tails are less than 3M on a TN system then no RCD protection is required.

 

[horsleya]

So assuming that I install an additional RCD on a TT based system then I am assuming that this should be time delayed for discrimination and probably 100ma.

As there is little room at the service head is there a particular model of RCD that you would recommend or would you simply find a small IP rated CU with a standard din rail mounted RCD?

I am unsure what would constitute the appropriate level of insulation. I could use flexible conduit but how could I establish a good connection with the Meter as there is no way of obtaining a satisfactory seal.

Which approach do most people take? RCD or insulation for Tails in a TT system?
Sorry if these questions are a little basic but I am hoping to use my own TT based system as a CU upgrade for an NIC assessment.

 

[Sintra]

Basically what i meant in the last post was on a TT the only time you need a RCD on supply tails is when the tails go to a metal clad consumer unit or isolator. If the enclosure is plastic then no RCD is required.

If however you have to install an RCD then you use a 100mA time delayed.

Something like this in a PVC enclosure.

80 Amp 100mA Time Delayed RCD - MK

 

[horsleya]

Ah that makes sense.

So no additional RCD required - Good news.

Thanks Sintra.