ESP Surge Protectors

I’ve used these for years whenever I find them on the surplus market. These are really good surge protectors. They often have a label saying not to remove them — they are usually deployed on installation of fancy leased office copy/scan/print/fax machines costing kilobucks and are required as part of the service agreement.


The green LED on this one is just a power on light. Note that some units have a safety relay inside and will not power on unless a good ground is present. They’re idiot proof. You’ll see momentarily why the ground is very important.


All of them can be opened without damage. On this one you pry from the back and carefully release locking tabs.


That… Uhh… That’s a static sensitive warning. I’m guessing that was some engineer’s joke as there are no static sensitive parts in this unit at all.


Here’s the good part. Surge protection on the inlet – line and neutral first meet these MOVs and gas tubes. Now this unit is kinda interesting in that it’s wired like this:

Line in —-{two MOVs in parallel}—[gas tube]—ground—[gas tube]—neutral

Normally I see the gas tubes and MOVs in parallel.

Not gonna question it; it works.

Now those are just the castle gates. There are more defenses beyond…


L network filters in line and neutral. Each must flow through an inductor on its way to the equipment. Once it passes the inductor which will resist the passage of both high frequency gibberish and spikes with fast rise times, a capacitor damps out the high frequency components. If you’re curious as to how this works, go deeper down the rabbit hole and research basic filter design. It starts to get quite fascinating.

One cap goes from line to neutral, two go between L/G and N/G. One resistor up before the inductors will bleed residual charge off these upon unplugging for safety.

This is a feature not present in most AC powered smoke alarms. Don’t touch their pins after disconnecting them. They bite.


Additional MOVs are provided here and are behind 2 amp pico fuses. Not sure why the fuses are used, but for a surge to hit these, it would have had to get through the primary defenses.


Telco protection stage; this uses poly switch fuses for current limiting (self resetting) and MOVs to limit voltage.


The MOVs can even be replaced if they pop.
Cool, right? I’ve never had any gear get damaged plugged into one of these. They Just Work.

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