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The Downing Street problem

AJJewsbury
No, nothing to do with politics - just earthing and bonding - just recent events have perhaps given us an interesting example.


As I'm sure everyone has noticed of late, No.10 Downing Street has a nice electric lamp for its front door - supported by an arch from the iron railings. Similarly, immediately next door to the left (contrary to usual UK practice, designated No.11) also has lights fixed from the railings - two in this case, at least one of which appears to share the same railing system as No.10.


If pictures help: No.10 and No.11


For the sake of this discussion let's assume No.10 and No.11 have separate DNO supplies and the light fittings are all Class I.


If you had to do the design for feeding the lights outside either or both premises (and had no ill-intent to the occupants) what sort of earthing and bonding arrangements would you specify? (and would regulation 714.411.3.1.2 figure in your thinking?)


  - Andy.


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    what does that reg say



    That's the point of this post really - the regs say quite a few things - not all would seem appropriate for a situation like this - and then there's 'usual interpretation' like the presumption that the main 'equipotential zone' only needs to exist inside a building so often we don't think of bonding metalwork outdoors - which the regulations don't (clearly) say.


    I guessed that the first thought of many (at least those used to normal domestics) would be to earth the light as if it were any other class I device (so a c.p.c. of say 1mm² or even 0.75mm² if the final connection is done in flex) and not to worry about bonding anything 'cos it's all outdoors to outside of the equipotential zone anyway. You could probably find sufficient words in BS 7671 to justify such an approach.


    My worry then would be that the c.p.c could be pressed into doing doing the job of a main bonding conductor - i.e. carrying significant currents perhaps for a long duration (especially if there's PME in the vicinity) - which perhaps suggests we should bond the railings after all. 714.411.203's allowance to allow 6mm² is actually for earthing and bonding conductors for PME systems, not c.p.c.s, and in effect allows a reduction in size from the normal sizes in table 54.8 - so isn't really a minimum for c.p.c.s. In this case where the railings might be the only extraneous-conductive-part shared by two installations - such a reduction doesn't seem obviously sensible, so perhaps we should bond with full size bonding conductors (as per table 54.8 if the installations are PME) after all.


       - Andy.
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  • 0

    what does that reg say



    That's the point of this post really - the regs say quite a few things - not all would seem appropriate for a situation like this - and then there's 'usual interpretation' like the presumption that the main 'equipotential zone' only needs to exist inside a building so often we don't think of bonding metalwork outdoors - which the regulations don't (clearly) say.


    I guessed that the first thought of many (at least those used to normal domestics) would be to earth the light as if it were any other class I device (so a c.p.c. of say 1mm² or even 0.75mm² if the final connection is done in flex) and not to worry about bonding anything 'cos it's all outdoors to outside of the equipotential zone anyway. You could probably find sufficient words in BS 7671 to justify such an approach.


    My worry then would be that the c.p.c could be pressed into doing doing the job of a main bonding conductor - i.e. carrying significant currents perhaps for a long duration (especially if there's PME in the vicinity) - which perhaps suggests we should bond the railings after all. 714.411.203's allowance to allow 6mm² is actually for earthing and bonding conductors for PME systems, not c.p.c.s, and in effect allows a reduction in size from the normal sizes in table 54.8 - so isn't really a minimum for c.p.c.s. In this case where the railings might be the only extraneous-conductive-part shared by two installations - such a reduction doesn't seem obviously sensible, so perhaps we should bond with full size bonding conductors (as per table 54.8 if the installations are PME) after all.


       - Andy.
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