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.

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.


 

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.