It is probably obvious but would appreciate your thoughts.

I have a system that is about 5 years old, I started to get some false alarms that have puzzled me.

By process of elimination and checking I have discovered that the wiring into the exit door magnetic contact had gone green with damp. Although securely wired would this cause an intermittent problem, I would have thought it would knock it out completely. The alarm is set for 75% of the 24 hr day and is false alarming on average 2 times a week. Obviously time will tell but I thought I would draw on your knowledge.

p.s. great site.

I've seen this problem quite a few times, normally near the sea!

Yes, it does cause a problem.

The corrosion will eventually cause a permanent fault, but in the meantime it will just gradually increase in resistance as it corrodes, causing the false alarms to become more frequent.

Changing the contact and trying to damp proof it would seem a matter of priority.

You might also find the wiring corroded some way back also!

Thanks for those replies.

Can I damp proof it by putting a little vasalene on the terminals on the magnetic contacts, a bit like greasing car battery terminals?

As for corrosion going back down the wire, as long as one gets to "clean" wire that should be ok? In my case only a centimetre or two.

Thanks.

You could use some Epoxy resin, or perhaps some silicon sealant instead of vaseline.