On Oct 16, 1996 at 18:07 hrs, Arthur Marsh of 3:800/812 wrote to John Clarke:

Hello Arthur,

JC>> Another problem is that it is not easy to determine if the 
JC>> arrestor/zener combination is till operational or has failed 
JC>> for some reason.  If this has happened, and you don't know 
JC>> about it, then you are no better off than having nothing at all.

 AM> Wouldn't a high voltage, low current power supply be able to trigger 
 AM> the zeners (exceding the zener breakdown voltage) without causing 
 AM> them to destruct? Then have two sets of the zeners, and automatically 
 AM> test them one at a time using a "make before break" switch so that 
 AM> even if there is a power surge during a test, there will be one set 
 AM> of zeners to carry the surge?

The usual way to test them is to use an adjustable high voltage low cuurent
power supply.  The voltage is gradually wound up and the voltage v current
graph plotted.  A working arrestor should have a graph that rises linearly
with voltage until the operating voltage is reached.  At this point the
voltage drops slightly with an increase of the current to the maintain
voltage.  The voltage then stays constant with rising current.

JC>> Perhaps the best arrangement is to also include a series 
JC>> fuse that blows when the arrestor operates.  This way you 
JC>> know when the arrestor has fired since the equipment won't 
JC>> work until the fuse is replaced.  At the same time the 
JC>> arrestor/zener combination could be checked to see if 
JC>> healthy and replaced.

 AM> True - I'd like to have something like this before using computer and 
 AM> communications equipment in the tropics.

Not just the tropics - anywhere where electrical storms are likely.

Regards ... John
@EOT:

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