Whats in an SPD?

[spynage]

Hi.

Wondering whats in an SPD that makes such a price difference between manufacturers? For instance a British General single pole SPD retails for about £20. Wylex £60+ Do they work to the same standard like RCD's? Any way of testing them?

Thanks

S

 

[littlespark]

An spd as you know is a surge protection device. Takes thousands of volts of lightning strike instantaneously and sends it live to neutral, rather than through your appliances and devices.

the price difference I can only assume is how many of these surges it can take before failing.

there are classes of spd as well, depending on whether overhead supply to house, likelihood of lightning strike, etc

 

[pc1966]

As above, they conduct on over-voltage and dissipate most of the surge energy. Their cost is partly down to quality of construction & brand-loyalty, but also how much energy the component part can absorb before it fails.

Most home only really need a Type 2 SPD, one that is rated for the rather short surge that is induced from a nearby lightning strike or a big fault / motor switching, etc.

However, if you are at the end of a long overhead line, or if your property has a lightning protection system, then you might be faced with a much larger surge from a direct hit on something conductive and so a Type 1 (or Type1+2) would be needed. They are way more expensive as they can absorb an order of magnitude greater energy.

Forget if it is 10kA or 20kA peak rated SPD, the difference between a 8us/20us shape and a 10us/350us shape is far bigger!

You can test them with an insulation tester, though some of the fancier MFT have a specific test option to do so. Basically if you check at 250V DC they should not conduct (resistance over 10M) but attempting a test at 500V (for type 2) will show quite a low IR value (well under 1M). Many IR testes show the sustained test voltage during the test, that typically would be in the 400V region.

Some Type 1 don't really show any conduction until you try at 1kV when the gas discharge tubes flash over, but that is risky to do on an installation in case something is still connected and doesn't like it!

 

[Julie.]

There are two basic designs.

The first uses a spark-gap and a resistance - when the voltage raises above the spark-over value the spark conducts, and the resistance limits the current.

The second uses a voltage dependent resistor - the trade name used to be Metrosil - this exhibits a high resistance until the voltage goes above a certain level at which the resistance drops rapidly.

Each unit may have one of either L-N and L-E , some use different types, others use the same for both.

I think the VDR type is better from a system point of view as there is a resistance transition between the two states, the spark-gap is cheaper but leads to a sharp change, and of course doesn't cut-off at the same voltage as triggering unlike the VDR which does.

Whether it makes any reel difference - I don't know.