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Metal CU, TT earth and wiring for SPD

Former Community Member
Former Community Member

It appears from everything that I have read that for a TT system it's normal to use a metal consumer unit with normal 100A main incoming switch and RCBO's for circuit protection.

This is based on the unprotected live conductors, meter tails and busbar being well protected and supported reducing the risk of fault. Therefore the risk caused by high earth loop impedance and a live conductor contacting the metal consumer unit is very  low.

It also from catalogues etc to be normal practice to introduce SPD's into these CU's along with a circuit breaker to protect them. This means additional wiring which presumably is not double insulated and does not benefit from additional support.

I could avoid the above with an external SPD but that adds cost and just makes the installation more cluttered.

Am I correct that industry believes it's acceptable to use spd's internally on TT protected CU's along with the additional risk the wiring would cause?

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    Do please note that SPDs for systems with a TT supply will normally include a gas discharge device (spark gap in a controlled atmosphere ,usually a ceramic pellet) These when they fail, fail  open circuit, unlike the solid state devices that can often fail short. On a TNx set-up with a low Zs, the short circuit failure from live to earth is OK, as the fuse opens. On TT, unless the electrodes are unusually good, this will not happen, and the dead SPD just makes everything live, and nothing trips.

    The solid state arrestors are rather better in terms of less of a voltage excess  between no break-over and always break-over voltages,  and do not follow through like an arc can - once struck the voltage has to fall very low for an arc  to go out. 

    so SPD units for TT and TNS look different inside, as the TT one will have a gas discharge tube in the earth path.

    For TT (from here) The thing with the arrow heads represents the gdt = spark gap.

     

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  • 0

    Do please note that SPDs for systems with a TT supply will normally include a gas discharge device (spark gap in a controlled atmosphere ,usually a ceramic pellet) These when they fail, fail  open circuit, unlike the solid state devices that can often fail short. On a TNx set-up with a low Zs, the short circuit failure from live to earth is OK, as the fuse opens. On TT, unless the electrodes are unusually good, this will not happen, and the dead SPD just makes everything live, and nothing trips.

    The solid state arrestors are rather better in terms of less of a voltage excess  between no break-over and always break-over voltages,  and do not follow through like an arc can - once struck the voltage has to fall very low for an arc  to go out. 

    so SPD units for TT and TNS look different inside, as the TT one will have a gas discharge tube in the earth path.

    For TT (from here) The thing with the arrow heads represents the gdt = spark gap.

     

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