The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 21 replies
  • Subscribers 31 subscribers
  • Views 2988 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Metal CU, TT earth and wiring for SPD

Former Community Member
Former Community Member

It appears from everything that I have read that for a TT system it's normal to use a metal consumer unit with normal 100A main incoming switch and RCBO's for circuit protection.

This is based on the unprotected live conductors, meter tails and busbar being well protected and supported reducing the risk of fault. Therefore the risk caused by high earth loop impedance and a live conductor contacting the metal consumer unit is very  low.

It also from catalogues etc to be normal practice to introduce SPD's into these CU's along with a circuit breaker to protect them. This means additional wiring which presumably is not double insulated and does not benefit from additional support.

I could avoid the above with an external SPD but that adds cost and just makes the installation more cluttered.

Am I correct that industry believes it's acceptable to use spd's internally on TT protected CU's along with the additional risk the wiring would cause?

Parents
  • 0

    Looking at the Wylex RCD device from TLC that Chris pointed out, its in a polycarbonate, not metallic case. My understanding that under regulation 421.1.201 this device would be considered as a similar device and would need to be non-combustible in domestic environments? 

    Quite a few would argue that it's NOT “similar switchgear" - likewise a simple RCCD in an insulating enclosure. The definitions of a consumer unit, distribution board (of which a CU is one case), and switchgear all involve distinct incoming and outgoing circuits (hence must contain overcurrent protective devices, by the definition of a circuit) or main and auxiliary switching equipment (e.g. main switch and separate controls for outgoing circuits) in the case of switchgear.  If we start over-interpreting the definitions we'll end up thinking we need metal clad cooker control units, shower switches, henley blocks and FCUs.

       - Andy.

Reply
  • 0

    Looking at the Wylex RCD device from TLC that Chris pointed out, its in a polycarbonate, not metallic case. My understanding that under regulation 421.1.201 this device would be considered as a similar device and would need to be non-combustible in domestic environments? 

    Quite a few would argue that it's NOT “similar switchgear" - likewise a simple RCCD in an insulating enclosure. The definitions of a consumer unit, distribution board (of which a CU is one case), and switchgear all involve distinct incoming and outgoing circuits (hence must contain overcurrent protective devices, by the definition of a circuit) or main and auxiliary switching equipment (e.g. main switch and separate controls for outgoing circuits) in the case of switchgear.  If we start over-interpreting the definitions we'll end up thinking we need metal clad cooker control units, shower switches, henley blocks and FCUs.

       - Andy.

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines