Surge protector monitoring

[radar_macgyver]

I've recently put together a couple of industrial enclosures where I used some type-3 surge protection devices like these:

https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=2905229

They all have an in-built monitor that lights green when the protector is OK, and red when it needs replacement. They also have a dry contact output that goes open when the protector needs replacement. I understand that these are MOV-based, and have a limited life, but I'm curious how the monitoring circuits work. Anyone here knows?

[BradC]

The MOVs are protected with fuses. When a fuse opens the device reports it needs replacing.

MOVs have two failure modes. Progressive damage causes the device to lower in resistance. This causes heating and eventual opening of an associated thermal fuse in the circuit. This fuse can be integral to the mov, or external in close proximity. This is the desired and usual failure mode. Each time the device clamps a spike it wears out a little more, until it fails low resistance or short.

The second failure mode is a massive spike causing catastrophic failure. In a lot of cases this fails open and does not trigger the protective fuse or respective monitoring circuit. It's a lot rarer than the fail short failure mode though, and it takes a Big Bang to make it happen. (Love the way iOS auto-correct capitalises Big Bang).

[radar_macgyver]

Thanks, Brad. I guess that's why the modules can be expensive, with all that monitoring circuitry in there.

If one follows Phoenix Contact's design guide, there ought to be a type 1 (GDT) or type 2 (larger MOV) protector ahead of the type 3, which would absorb much of the energy from a bigger bang. I was surprised to see that the type 1 protectors are rated for a direct hit - didn't think anything could survive that.

[BradC]

I was surprised to see that the type 1 protectors are rated for a direct hit - didn't think anything could survive that.

Me neither. I'd want to see proof. I've seen too many surge protectors rendered into a block of charcoal with a direct hit. One of the sites I'm working around (in the North West of Australia) at the moment gets 60-80 direct strikes in a good storm. The record so far was 88 in an hour. Even if the first suppressor survives a couple of hits, that sort of barrage is pretty overwhelming.

What makes it harder is there is about 400 suppressors on the ELV systems, and none of those have indicators. We're in the process of developing some high voltage test equipment and a methodology to allow individual testing of each device after a big storm, as any decent bang just renders the surge suppression componennts (GAS, MOV & TVS) open circuit. So no indication of failure, but no protection. At least all the power protectors have indicators (for what they are worth in a direct strike scenario).