TT System

Discussion in 'Electricians' Talk' started by Jakalama, May 21, 2020.

  1. Jakalama

    Jakalama New Member

    Hi
    New to this forum but hoping someone can help me.
    I’ve been working for myself for 6 months now and never come across a TT system.
    I’ve installed a new consumer unit in a pool plant room (domestic size pool)
    On completion of the work I carried out the testing and all was fine until I got to Zs at DB.
    I got a result of 90 ohms and a fault current of just 2.25A.
    I thought I would test at the main DB thinking the problem was with my installation but I got very similar results testing the main earth. can someone please give me some guidance on what I should be seeing from a TT system for PFC.
    I know resistance should be ideally 21 ohms or below but can be up to 200 albeit unstable. I’ve tested many consumer units and never encountered this issue before.
    Photo of the main DB cupboard incoming supply etc beliw
    Cheers
    Ray
     
  2. Jakalama

    Jakalama New Member

  3. Coloumb

    Coloumb Screwfix Select

    You can't get the Zs low enough in a TT to make it worth bothering with. I've not ever had to work with TT but my understanding is there should be a 100ma time delay RCD protecting the whole installation at the origin. If this CU is feeding equipment for the pool then there are a whole raft of other regs you need to take care of.

    Like I say, I'm no expert, but did it not occur to you to at least find out what the earth type was before you started the job? I would have thought it would be the least you could do.

    Edit sozza I see it's the PFC your after.
     
    Last edited: May 21, 2020
  4. unphased

    unphased Screwfix Select

    TT earths are poor. This is why 30mA RCDs are so important. 90ohms is not bad tbh. You will rarely, if ever, get a reading off a rod as low as 21 ohms. Ohms Law dictates your PFC. As long as you have a 30mA RCD on your circuits it's fine.
     
  5. Jakalama

    Jakalama New Member

    Unphased, thank you puts my mind at rest as all DBs have 30mA RCDs
     
  6. Lectrician

    Lectrician Screwfix Select

    You should have an upfront RCD of 100mA, or even 300mA or 500mA. This would be a time delayed type. You would then put 30mA RCD protection were needed. If all your circuits have 30mA protection, there’s no need for an upfront RCD unless you’ve run a submain or similar?

    As for PFC, you would record the L to N or P to P value, as this would be the highest.
     
  7. unphased

    unphased Screwfix Select

    Lec, I just need clarification on time delayed RCDs if I may. :)

    When the old style split load boards were in fashion, when they were used on a TT earth it was traditional to have a 100mA time delayed RCD as the main switch and 30mA RCD on the split. As things progressed and 30mA RCD was required pretty much on all circuits, the dual RCD board came in and the need for a time delayed RCD main switch disappeared. However, that was when we had plastic boards. Now that metal boards are in vogue there is a small risk of main tails shorting on the chassis so it is advisable to have an upfront 100mA time delay RCD on the tails before they enter the metal enclosure. Am I correct in thinking that the time delay element is purely to ensure discrimination with the down stream 30mA RCDs in the event a fault occurs?

    Similarly, If you have a sub-main cable supplying any switchgear it is usual not to have any RCD protection at the front of the sub-main. But, if TT earth is in use would it be advisable to have a 100mA time delay RCD on it for fault protection should the cable get damaged? Or am I right in thinking you don't need to. 5s disconnection time applies and no need for 30mA protection as its purely for ensuring disconnection in the event of damage to the sub-main, not life protection.
     
  8. Coloumb

    Coloumb Screwfix Select

    I can't see how you could avoid fitting a 100ma time delay RCD to a sub-main as you would have to install it so as to be compliant with automatic disconnection, ie the fuse would cover p + n faults and overloads but not faults to earth, which liven up anything connected to earth.
     
  9. Lectrician

    Lectrician Screwfix Select

    The time delay is to ensure discrimination is achieved.

    With metal CUs or DBs, it was always important to include upfront RCD protection in an insulated enclosure. When metal CUs became common place in domestics with 30mA RCDs or RCBOs in them, upfront RCDs were originally used, but the NICEIC and manufacturers re-interpreted the regs and decided as long as you reduce the risk to minimum of a fault on the tails, you can omit the external upfront RCD. To minimise the risk, the use of the "Tails Glands" is deemed as required. People use them on TN systems too, but they were meant for TT. Hagar also included clamps for the tails.

    With a submain as opposed to tails, an upfront RCD is essential.

    A 5 second disconnection is acceptable for the submains, achieved through a fuse on a TN system, or a 1 second disconnection time achieved with an RCD on a TT system.
     
  10. unphased

    unphased Screwfix Select

    Thank you Lec for the explanation.
     

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