Surge protection on an mcb on a TT

That sounds wrong, all the live conductors running around the consumer unit relying on the suppliers fuse is not good. Swap out the main switch for a time delayed rcd. 

In principle I'd be happy about an SPD before an RCD on a TT system - that way you don't have a delicate RCD exposed to the full surge currents. Everything before the RCD should be protected by double/reinforced insulation however - so I'd expect the wiring between the MCB and SPD to be at least insulated & sheathed. Single insulation where it can touch the inside of steel box where ADS won't function isn't my idea of good.

Strictly speaking the internals of the CU would conform to its product standard rather than directly to BS 7671's concepts - but I'd suspect they're not so different in practice on this point.

   - Andy.

I’ve questioned this before and then told myself that the SPD isn’t protected by an mcb, it’s just supplied via one. It seems to be the norm in many preloaded units though, iv fitted quite a few and round here there’s a lot of TT systems, from my point of view I’m using the good old get out of jail free card that is the manufacturers instructions.

As it’s not part of a final circuit the risk has got to be fairly small, it’s all in a fire proof earthed box very close the origin of the installation after all.

What happens if the S.P.D. goes dead short L to E in a TT system?

www.bing.com/.../search

Z.

"In principle I'd be happy about an SPD before an RCD on a TT system - that way you don't have a delicate RCD exposed to the full surge currents."     A.J.                                                                                                               High Voltage surges can originate from within installations and don't always come from outside.

https://www.youtube.com/watch?v=eq8mKTaBDPY

Z.

"That depends what exactly is in the SPD, the metal oxide based devices have a distressing tendency to fail dead short at end of life, but many makers in effect include a series fuse wire as part of the design." mapj1                                                        https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=106261

Z.

Voltage surges can originate from within installations and don't always come from outside.

True, but internally generated surges (e.g. from switching) are normally much less powerful than some external ones (typically lightning related). Even BS 7671 only considers lightning causes in its risk assessment for whether SPDs are required or not. Nothing to stop you specifying type 3 SPDs within the installation to protect vulnerable equipment if the need arises, but you'd probably still want a type 1 or 2 as close to the origin as you could.

it’s all in a fire proof earthed box

The fact that the box is earthed if anything makes the problem worse where the high earth fault loop impedance means that ADS won't occur. An earth fault from that connecting wire won't just make the CU enclosure live, but via bonding and c.p.c.s all exposed- and extraneous-conductive-parts of the entire installation would also be made live - and not just for a few seconds, but indefinitely. Would you be comfortable if the metal CU case was left unearthed?

What happens if the S.P.D. goes dead short L to E in a T system?

SPDs pre-RCD in a TT system should never be wired L-PE - they should be "CT2" - i.e one element L-N and another N-PE. If the L-N fails dead short it's a L-N fault and the upstream fuse blows promptly even in a TT system, if the N-PE one fails it's not nice but far less hazardous. Usually the N-PE one is a gas-discharge type which is less likely to fail to short anyway.

   - Andy.

CT1 is a TT system with S.P.D.s connected  L to Earth.

CT2 also has S.P.D.s connected L to E, albeit with two devices in series.

What happens if Ra is high. Will the installation see dangerous Voltages on metalwork if an S.P.D. shorts out?

At what Voltage do gas discharge tubes start to operate to protect equipment?

I believe that gas discharge tubes work slower than diodes. Is that true?

www.youtube.com/watch

Z.

CT1 is a TT system with S.P.D.s connected  L to Earth.

Only CT2 is permitted upstream of the first RCD on TT systems - reg 534.4.6.

CT1 can be used on TN or only after an RCD on TT systems.

   - Andy.

Thanks for the replies, thinking about it the Hager model is fed straight from the main switch, in some ways this is better as the loop is very small i think and can't reach the metalwork. Which begs the question why the mcb anyway ? The board i used here was fusebox.

Gary