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TT CT2 SPDs and TN requirement

wallywombat
534.4.6 says that for TT systems where an SPD is upstream of the RCD, then a CT2 SPD must be used (fair enough), but also says that "the conditions of Regulation 411.4.1 shall be met". But 411.4.1 is about TN systems having a dependable connection to earth. In particular:
411.4 TN system

411.4.1 In a TN system, the integrity of the earthing of the installation depends on the reliable and effective

connection of the PEN or PE conductors to Earth. Where the earthing is provided from a public or other supply

system, compliance with the necessary conditions external to the installation is the responsibility of the distributor.

I don't get why a section about SPDs in a TT system refers to a TN requirement.
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    wallywombat:

    534.4.6 says that for TT systems where an SPD is upstream of the RCD, then a CT2 SPD must be used (fair enough), but also says that "the conditions of Regulation 411.4.1 shall be met". But 411.4.1 is about TN systems having a dependable connection to earth. In particular:
    411.4 TN system

    411.4.1 In a TN system, the integrity of the earthing of the installation depends on the reliable and effective

    connection of the PEN or PE conductors to Earth. Where the earthing is provided from a public or other supply

    system, compliance with the necessary conditions external to the installation is the responsibility of the distributor.

    I don't get why a section about SPDs in a TT system refers to a TN requirement.


    Section 534..4.5 deals with Protection of the S.P.D. against overcurrent.  Most modern S.P.D.s have built in overcurrent protection. So if they go bang zap pop inside they will disconnect themselves from the supply. Then hopefully an indicator of some sort will alert us when we venture into the cupboard under the stairs and open the lid of the consumer unit. Then the expensive replaceable S.P.D. module thingy can be removed and replaced.


    534.4.6 is concerned with fault protection remaining effective after the S.P.D. has gone bang. So the installation earthing and/or R.C.D function in TT systems is necessary to maintain safety even after the failure of the S.P.D.


    That is how I understand things.


    P.S. ............534.4.6 requires shock protection to be maintained in the case of a faulty S.P.D.


    (a)  says that in TT systems this can be provided if the S.P.D. is downstream of the R.C.D.


    OR 


    (b) with an S.P.D. upstream  of the R.C.D.  if the S.P.D. fails and shorts N to P.E. the requirements of 411.4.1 must apply.


    See Fig. 16A3 in appendix 16.


    If the S.P.D. that is situated before the R.C.D. in a TT system shorts N to E, then this converts the TT system to a C.N.E. system which may present danger in certain installations. See routes 4a and 4b.



    Z.


     


  • 0
    Yeah I understand all that, but i still can't see why 411.4.1 is made an additional requirement. Or to put it another way, suppose I have a TT system which compiles with all the relevant bits of BS 7671 - so there's an earth rod, 30mA RCD at origin etc. Then later, I connect a CT2 SPD upstream of the RCD. What (if any) extra constraint does 534.4.6 impose upon the installation (that wasn't already there by virtue of being a TT system), by requiring me additionally to comply with 411.4.1? And if it doesn't impose any extra constraints, why require it?
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    I agree it's not at all clear what it's trying to say (it's obviously confused me too as my copy has a pencil comment of 'Eh? 411.4.1. is about TN systems' against it - presumably from my first reading of it about three years ago).


    I suspect that Z. has  a handle on it - i.e. when the N-PE SPD shorted, the system in effect becomes TN-C-S and so the supply N is acting as a PE - so the supply N should meet the same requirements for earthing as the PE or PEN in a TN system. Otherwise any nasty voltages on the supply N get imposed on the consumer's exposed/extraneous metalwork.


    Does perhaps rather beg some awkward questions where the distribution system doesn't meet PME standards though...


    Or it might be a typo and it should be referring to something else completely.


      - Andy.

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    Perhaps in view of it's doubtful usefulness and the apparent vagueness as to it's correct installation, then maybe BS7671 should allow an exemption for use.

    I have a job coming up on a TT supplied installation whereby a new consumer unit will be fitted in addition to the existing set-up. I wish to elect to use a CPFUSEBOX item with a T2 SPD and main switch. The unit comes with a B32A mcb to protect the SPD and the subcircuits will be covered by individual RCBOs.

    The only info I can see is that their SPD is labelled T2.
  • 0
    Note that T2 and CT2 are two different things.
  • 0
    whjohnson:

    :

    I have a job coming up on a TT supplied installation whereby a new consumer unit will be fitted in addition to the existing set-up. I wish to elect to use a CPFUSEBOX item with a T2 SPD

    :

    The only info I can see is that their SPD is labelled T2.


    If it's this one https://www.consumerunitworld.co.uk/fusebox-spd2ptn-type-2-surge-protection-device-3644-p.asp the description says suitable for TN - so I'd suspect it's configured CT1 (with the bottom of the L SPD connected to PE rather than N) - which means it wouldn't be suitable for connection in TT system upstream of the first RCD.


       - Andy.


  • 0
    This all hinges on the failure modes of the solid state surge arrestors, which tend to fail to a low resistance (near short) condition. These then need a series fuse or thermal cut-out or similar which will only blow if there is enough current available. On TT there may not be if Zs is not low enough.,


    Gas discharge surge arrestors tend to fail in the sense that either the electrodes burn back, or the gas escapes,  and both of these result in the striking voltage eventually rising so far they do not operate at all.

    A gas discharge device is not something to be left accross the mains however, as once struck by the impulse of a few kV it will stay lit, like a neon lamp without the limiter resistor, or like striking an arc with a welder - once the arc has struck it needs the voltage to fall enough for it to extinguish. If there is an unlimited source of current at low voltage available it will overheat and fail in short order.

    Hence the slightly more complex internals of the TT type surge arrestor with both types of device.

    The regs do not explain this at all, and make the description of  how to do surge protection far more complex than it needs to be (and ambiguous...).


    Mike.
  • 0
    So on a TT system, if you have a CT2 SPD upstream of the RCD - i.e. with varistors between lines and N, and a gas discharge SPD between N and PE - is there anything that could go wrong which would require in some way a "better than usual" TT supply?
  • 0
    In principle if the varistor blew SC, there is plenty of LN current, so the fuse will remove it. (or it will remove itself, depending on the design) Steady state once the event has passed N and E are at nearly the same voltage and the arc goes out in the gas, all is well.


    During the zap, the TT earth electrode and wiring will rise quite a way above terra firma earth, perhaps more than a TN system, but to be honest  perhaps not, depending on the wiring inductances.

    The situation is not clear cut.

    M.
  • 0
    It sounds to me that these devices are of limited value and perhaps cause more problems in terms of making an installation potentially unsafe with their presence than real use.

    In the event of a L-N or LE short within the device, I thought the purpose supplied and connected upstream mcb would negate the large fault current, which I thought was why CPFUSEBOX began to include mcbs in their 2nd series of consumer units. The initial consumer units did not have a purpose installed mcb, but CPFUSEBOX stated that any non-mcb equipped units did not require any retrofitting.

    I thought the principle difference between CT1 & CT2 devices was in their ability to handle differerent sized fault currents, with the CT1 device being rated for higher levels than CT2..

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