surge prot. circuit:how does this work?

The green cylinders are gas-filled devices which also have over-voltage protection characteristics.

Very nice job with the photos. This protector seems a cut above the typical consumer device. The blue part labeled C1 and C1 would be capacitors. These are generally placed there to suppress RF (radio frequency). The parts labeled RV2 and RV1 would be MOVs (metal oxide varistors) These are the part that does the over voltage protecting. The remainder of the devices will be associated with protection for a telephone line.

These devices protect against brief transient voltage conditions (voltage "spikes"). They are rated in how many Joules of energy that they can convert to heat before they destruct.

This unit is most suitable for use with computers, copiers, fax machines, and other small electronics.

Here are some ratings of assorted MOVs

I believe the blue devices *are* caps. The yellow ones are probably MOVs (metal oxide varistors). these show a high resistance until a certain voltage peak is reached. They then abruptly change to a low resistance, shunting a voltage spike to earth (ground). The green tubular things are gas discharge devices that will conduct at a certain voltage.

This sort of "surge suppressor" is not very useful since the earth wire is not usually close to a "real" earth. The will protect against line to line spikes, but most devices are pretty immune to these. In addition the MOVs appear, from their size, to be limited to about 70 Joules.

The black "transformer looking thing" is probably a ferrite line filter. It might be useful for a radio or TV to eliminate line, (mains), conducted interference.

I tied using Google to look for ISDN SP-6P with no results I could read.

It's always fun trying to figure out what things do! ;-)

Virg Wall