SOT: surge protection

[Scott]

If the surge light is green, does this mean an extension lead is working okay or does it mean a surge has taken place? Do these units last for ever or are they destroyed if a major surge takes place?

[alan_m]

The surge protection devices may have a limited working life ( x number of surges)

The lights can be meaningless.

[Scott]

Interesting. What counts as a surge? Do voltage fluctuations count or does it have to be a proper lightning strike? Is there any way of knowing if it is life expired?

[Theo]

Here a consumer-unit SPD contains a metal oxide varistor which is normally high resistance. When a high voltage is applied, it will go low resistance and conduct it to earth. When the MOV has absorbed too much energy it becomes end of life. Its normal resistance will decrease and it'll melt some solder to trip a spring that displays an indicator (eg a red window on the device).

What the lights on your power strip might mean rather depends on how it is designed. They could indicate a MOV failure, or just be for decoration, who knows.

Theo

[John Rumm]

No idea. What extension lead are you talking about? Do you have a link to the product in question? Have you looked for a manual?

Most "surge protected" extension leads contain one or more Metal Oxide Varisters (MOV). These present a high resistance at normal mains voltage, but it falls at higher voltages allowing the spike to be shunted to ground or neutral.

So if you take a product like this:

It can dissipate up to 65 Joules over 2 milliseconds when at its maximum ambient temperature of 85 deg C.

If you exceed those limits, then the device will be degraded or destroyed. So longer surges are more likely to cause damage than shorter spikes.

[Scott]

It is fairly nondescript and any packaging is long discarded.

I assume destruction would involve it not working at all. Would degrading make it more - or less - likely to 'trip' the next time? I suppose the answer is to use a dedicated UPS to protect against surges and outages.

[Theo]

There is no 'tripping'. It just silently conducts the surge away to earth. Eventually it wears out and then it doesn't. Which is why there's an indicator to tell you that it's worn out and needs replacement.

Theo

[Scott]

Which you previously said could be 'meaningless' :-)

[Theo]

I probably didn't add enough caveats :-)

(In a well-designed system like a consumer unit SPD) there's an indicator to tell you that it's worn out and needs replacement.

In a random power strip there may not be this kind of mechanical indicator, so it's up to the power strip manufacturer what the lights mean, or whether they mean anything at all. They may or may not have designed it properly, as in:

Theo

[Scott]

Thanks. I like Big Clive. He is very clear. I understand better now.

[John Rumm]

Yup, it is a shame he does not post here any more...

[Brian Gaff]

Yes well, I had a surge protected socket bar, and inside was a smalldisc shaped stripped device wired across the live and neutral. It had gone short circuit and blown the fuse. It was, I think some form of VERY that went short circuit if the voltage got above a certain level. I got a new one I think from CPC and shoved it in and it was fine, I guess until the next surge. It was far more simple than I suspected. Indeed these used to be used in some old TVs merely to protect them when they were far less reliable. I imagine the ones with lights must have some active circuitry in them, and I wonder if they are as fast? Brian

[Brian Gaff]

It seems to be something like a voltage of over double the mains voltage in the main, however generated. Nothing will protect your stuff if the wiring gets a direct hit from the lightening. , I have seen the results at one of the buildings where I worked Sockets propelled across the room and burned and chard cables and plugs. Usually the equipment is totally unrepairable unless you were lucky. Brian

[Animal]

It's easily solved, just pull out the surge protecting device & your pointless fire risk is gone.

[alan_m]