TT and double pole

Discussion in 'Electricians' Talk' started by Jako202196, May 16, 2022.

  1. Jako202196

    Jako202196 Member

    Hi guys what’s the regs on double pole isolation for a TT earthing arrangement? I have a TNCS earthing arrangement where a hot tub has been installed. The hot tub is fed by a 32amp type c rcbo single pole then goes to a local isolator where the TT island is made. Is the 4 polar isolator sufficient in this case?
     
  2. MGW

    MGW Screwfix Select

    This may help the use of TN and TT is a problem as distance matters, we are looking at the voltage gradient under fault conditions, you can get duel pole switching RCBO's but over load only sensed on the line, not neutral.

    Where TN-C-S is banned in the main fire regulations result in a minimum distance, I am thinking about caravans, with TT both lives (line and neutral) do need switching, and the RCD is the primary protective device rather than secondary, so type matters much more, I am happy with type AC with a TN supply, but would insist on type A with a TT supply.
     
  3. The Happy Builder

    The Happy Builder Screwfix Select

    I can’t see a problem with it, because we use single pole protection, we don’t install double pole fuses do we?

    Having said that personally I would generally install a double pole RCD for a hot tub than a just a single pole pole RCBO, but would not be worrying about having a single pole fuse or MCB.

    Either way the equipment can be isolated for electrical maintenance as required using a double pole isolation switch.
     
  4. Jako202196

    Jako202196 Member

    Thanks guys for your replies. So the requirements are set for maintenance?
     
  5. Bob Rathbone

    Bob Rathbone Screwfix Select

    Fuses and circuit breakers are for overcurrent protection and in some cases earth fault protection only. They do not provide isolation as they do not break all live conductors. This is basic stuff taught for NVQII.
     
  6. The Happy Builder

    The Happy Builder Screwfix Select

    In a TN-C-S earthed installation there could be as much as 30 volts on the neutral, but it should be tied down to earth so it should generally be less.

    In a TT installation the neutral is not tied down to earth, so may be higher.

    People assume that neutral conductors don’t have any voltage on them or current glowing through them, which is a dangerous assumption to make.
     
    Jako202196 likes this.
  7. The Happy Builder

    The Happy Builder Screwfix Select

    Which is why all installations have a main isolation switch upfront, the question is should that main switch be double or single pole?

    In a TN earthed installation removing the fuse isolates the circuit, in a TT earthed installation it doesn’t, you have to turn the main switch off, because the neutral has to be isolated in a TT installation although it doesn’t in a TN earthed installation. How many TT installations have you seen with double pole fuses?

    In this particular installation the actual installation main switch could be single pole, because the main installation is TN earthed, but then there has been a TT island created for the the hot tub so the hot tub must have a double pole isolation switch, which it has.

    If the RCBO trips because it functions as either a RCD or MCB it will not isolate circuit, neither will it isolate the circuit if it is turned off by hand, so the neutral stays connected, which is okay in a TN earthed installation but not in this TT installation, because of the increased shock risk from touching the neutral conductor.

    So when carrying out electrical work within the hot tub it must be isolated by turning its double pole isolator off, NOT the RCBO in the consumer unit.
     
  8. The Happy Builder

    The Happy Builder Screwfix Select

  9. The Happy Builder

    The Happy Builder Screwfix Select

    That question cannot be answered by looking at the picture of the MCB, because it depends on the earthing system as to whether it’s acceptable or not, it is okay in a TN earthed installation, but not in a TT earthed installation.

    However all domestic installations have to have a main switch that isolates all the live conductors, both the lines and neutrals.
     
  10. Bob Rathbone

    Bob Rathbone Screwfix Select

    Isolated OK for mechanical work not involving opening of enclosures, for instance, changing drive belts on motors or cleaning operations. Not safely isolated for electrical work when live conductors may be accessed due to the removal of enclosure lids or panels. Put the lock on the main switch as you have mentioned in a later post and safe electrical isolation is achieved for all work, mechanical and electrical. The Happy Builder clearly understands isolation, but my issue was with the mention of single pole devices and isolation.
     
  11. The Happy Builder

    The Happy Builder Screwfix Select

    I think that @Bob Rathbone and myself agree that the best advice for DIYers is turn the whole installation off at the main switch before starting any electrical work or if there's a problem.
     
    Bazza likes this.
  12. Jako202196

    Jako202196 Member

    I thought single pole isolation was an acceptable method of isolation on TN systems as stated in the electrical safety first document. I can understand this wouldn’t be the case in a TT system?
     
  13. The Happy Builder

    The Happy Builder Screwfix Select

    Single pole isolation is acceptable in TN installations, but domestic installations must have a multi pole main isolation switch

    All TT installations must have a multi pole main switch.

    The Wiring Regulations require double pole devices within some installations, for example caravans and motor homes should have both double pole RCD and MCBs.

    https://www.electricalsafetyfirst.org.uk/media/1201/best-practice-guide-2-issue-3.pdf
     
  14. Bazza-spark

    Bazza-spark Screwfix Select

  15. Bazza-spark

    Bazza-spark Screwfix Select

    So instead of proving your voltage tester on a current limited device, you prove yours on the incomer backed up only by the main fuse.

    Safe isolation procedure?
     
  16. The Happy Builder

    The Happy Builder Screwfix Select

    My proving unit is in the passenger footwell in my van should I need it and I doubt anyone else on this forum has more voltage testers than me, because I don’t trust others to actually wire things up correctly. ;)

    Approaching every job as though the people who worked on the installation before you were all idiots helps to keep you alive.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice