Surge protection on an mcb on a TT

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.

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.