SPD clamping voltage

[EW Zuber]

I used to work in the electronics repair field and have found many consumer electronics devices that had shorted (activated) MOV's typically rated for about 135 VAC. Problem is the clamping voltage was high enough that the electronics were usually severely damaged.

It seems that with the clamping voltages on the Midnite SPD's there will be nothing left of the electronics, (775 VAC clamping on a 120 VAC circuit) or maybe that is not their function. Are they more intended to prevent fires?

[boB]

The main function of the SPDs is for lightning protection.  Lightning is a common-mode surge.

That is, all wires coming into the device needing protection are raised in voltage relative to ground.

The idea is to keep the difference in all of those a low enough voltage so that the components' insulation
does not break down.  Equipment normally has a hi-pot test done to ensure that the insulation is good
(UL requirements) for some minimum voltage for safety reasons.   The SPD clamp V should be lower than
this voltage.  For instance, the Classic charge controller's hi-pot test from all main input and output wires
to ground is tested at 2,270 volts.  An SPD-600 should not let the voltage get much above 900 volts
or so (plus or minus...)

The SPD-300 is around 500 volts.

The SPD-115 is around 200 volts or so which would be around the 135 VAC you mention.

A few cycles higher than the clamp voltage will be fine, which is a "surge" but at just a high
voltage line condition that is sustained, it won't like that very much.  This is why for lightning
protection, the voltage is higher than nominal.  Lightning is a very short lived event and not really
anything like a sustained high line condition, luckily, so it can protect things rather well, usually.
And also, luckily for a common-mode surge, the internal components will usually live if
the SPD is strong.

That is, except maybe for a "direct" strike, where anything goes pretty much.  That can
be a tough one !

If you think about it, MOVs would have to be huge to dissipate the energy or continuous
power available at continuous high nominal line voltages.  I think that is why we see
those activated MOVs.  I've seen those in the past also.

A Constant Voltage Transformer (CVT) or good UPS along with SPDs and MOVs  would most
likely make a good solution where high line voltage and lightning might be expected.