Using zener diode to protect test circuit?

I am designing a test instrument that will be powered from the circuit under
test. Consequently there is a high probability of the incorrect voltage or
polarity being supplied to my circuit. I cannot protect against reverse
polarity by using a series diode as this will drop the supply voltage too
much.

An idea that I have seen (in the Microchip ICD1
http://makeashorterlink.com/?K388412FA) is to place a 5.1V zener diode
across the power input. This will short the supply if applied with the
wrong polarity and attempt to clamp it to 5.1V if too high a voltage is
applied. In either case the diode will draw a potentially large current
since a series resistor is not practical as that would drop the voltage
unacceptably under normal circumstances. However, this would probably work
if the test power supply were current limited.

If the zener did sink a high current and was destroyed, what would its
failure mode most likely be? Would it fail short-circuit or open-circuit? I
assume the latter, which would negate the protection.

Alternatively, is there an alternative approach that I could use?


.

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