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 8 replies
  • Subscribers 32 subscribers
  • Views 2508 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

TT system for garage, RCD question

Horace Horace
Hello, not used this forum in a while and have had to re-register. I hope someone can help. I have an upcoming job to replace an existing small consumer unit in a garage about 30m from the house. The supply at  the house is PME, and the current set up is a 4mm SWA fed from an MCB in the house CU,  I recommended a switch fuse but the customer does not want this as it means destroying the wooden cabinet the existing CU is in. So the feed to the garage will be from a 32A mcb..Ive specified an earth rod at the garage as its concrete floor.is damp and dont want to export the PME. I will earth the outer armoured wire via the gland at the house end only, not at the garage end, and provide a TT earth system to the new garage CU.


My question is regarding rcd's ? The 2 way garage CU will have x2 30mA  rcbo's for the outgoing circuits so then is there really a need for a 100ma main switch ? as it would serve no purpose. So to reiterate, the supply is SWA, protected by mcb at the house..If I use a 63A/100A mainswitch at the garage CU, will this be okay as it provides manual dp isolation., I want to reuse to existing garage CU and its metal.


The last time I installed an earth rod was over 20 years ago so want to
Parents
  • 0
    Well if the feed to this dist board is via 32A MCB then your Garage mainswitch is not needed to be more than that is it? You didn`t state what rating of the Garage RCBOs are but unlikely they`ll be more than 32A in total anyway. in fact with discrimination in mind any might be difficult to acheive. Personally I`d prefer the RCBOs to be double pole switching to acheive isolation of both live conductors as it`s a TT anyway (you can get such RCBOs in single module format).Ref the 100mA mainswitch, I`d prefer it to acheive cascading therefore a little resilience against RCD failure rates (note 100mA is not classed as personal protection though and also it should be a Time Delayed one). I`d prefer this 100mA unit to be at the feed end - not just to give the SWA RCD protection but to mitigate against RCD failure caused by "stiction" because it would offer different enviroments to cascaded RCDs. Whilst your concrete base would bring earthy potential into the Garage in reallity it would probably be as effective as an earth rod or plate etc in itself in reality, although yes it should be bonded to the Garage earth, Is there room at the house consumer unit to put a 100mA time delayed RCD?
Reply
  • 0
    Well if the feed to this dist board is via 32A MCB then your Garage mainswitch is not needed to be more than that is it? You didn`t state what rating of the Garage RCBOs are but unlikely they`ll be more than 32A in total anyway. in fact with discrimination in mind any might be difficult to acheive. Personally I`d prefer the RCBOs to be double pole switching to acheive isolation of both live conductors as it`s a TT anyway (you can get such RCBOs in single module format).Ref the 100mA mainswitch, I`d prefer it to acheive cascading therefore a little resilience against RCD failure rates (note 100mA is not classed as personal protection though and also it should be a Time Delayed one). I`d prefer this 100mA unit to be at the feed end - not just to give the SWA RCD protection but to mitigate against RCD failure caused by "stiction" because it would offer different enviroments to cascaded RCDs. Whilst your concrete base would bring earthy potential into the Garage in reallity it would probably be as effective as an earth rod or plate etc in itself in reality, although yes it should be bonded to the Garage earth, Is there room at the house consumer unit to put a 100mA time delayed RCD?
Children
No Data

Community Rules & Guidelines