Surge protection - monitoring effectiveness.

While I do not want to get into the politics of what should be in the wiring regs. (this time)

 

How do these things mitigate a surge when an isolation transformer is in cct? Surely the rise in energy in the primary is replicated to a degree in the secondary depending upon the turns ration? If this is the case, then surely there is still risk of damage to any electronics downstream?

How about the effects upon a step-up transfomer too?

Not really as you might have expected - the surge waveforms are very very short duration compared to the 50Hz mains cycle, and the magnetic core of a conventional power transformer is nothing like fast enough to respond (running out of steam somewhere in the high frequeny audio to ultrasonic regime depending on size), so it is as if the core is replaced with air. Also the capacitance between windings is far more significant than it would be for low frequency events, so really the magnetic coupling between the windings is more or less switched off, and the trasnformation ratio is more to do with the capacitance ratio on both sides, in the manner of an oscilloscope probe or a graded insulator.



(the capacitive voltage divider concept is illustrated in this article about scope probes) realise in a transformer it will not be anything like that simple, as the winding capacitance of the outgoing cables is very much distributed and may be better seen as sections of transmission line of varying impedances.

I investigated the aftermath of a surge for an insurance company. Estimated cost of damage £125K.

 

Sounds like loss of data server to a business to me.

It was only a sports centre. The surge burned out the emergency lighting CBS, 12 BMS outstations, numerous Variable Speed Drives, numerous luminaire ballasts. Some IT too. The Main LV Switchboard had unmonitored SPD which must have cleared a few surges in its life and was depleted. No other SPD was provided.



mrf

The sort of energy required to burn out one lamp ballast, let alone all the rest you describe, is not going to be stopped by a surge arrestor in a DIN rail RCD size enclosure, and if folk think it will, they are in for some serious disappointment.


To sidestep damage of that magnitude, you need something that puts a dead short on the lines and provides prompt disconnection before self destruction by blowing the supply or even the substation fuses. An enclosed spark gap with noble gas filling at the right pressure can be arranged to do that, but it  is not a small or delicate thing. (we put things like that on the lines to transmitter towers on hilltops)


However,  that sort of damage sounds much more like a lost neutral that no-one wanted to own up to - which again is not something a small SPD capable of dumping perhaps a couple of hundred joules will do much about - it probably will not conduct much even at 415 volts, and if it does start to it wont pull enough current to alter the out of balance situation, perhaps drawing an amp or two and then it will exit stage left never to be seen again after a tenth of a second or so once the transorb element has got hot enough.

Do they do the annoying beep as well?

Not as far as I know - I think they're simple volt-free c/o contacts. But as you say, you can then connect it to what you like...

   - Andy.