Re: OT Surge Protected Extension Leads - fail PAT test??!!??! Any ideas

["David Buckley" <db@xxxxxxx>]

--- In ukha_d@yahoogroups.com, "David Yeend"
<david.yeend@c...>
wrote:
> ... I am interested in knowing a little more about how
> such devices 'deal with' non-excessive surges and how
> they 'contain' excessive ones.

The way that voltage surges are contained is through the use of
Voltage Dependent Resistors, VDRs, sometimes called Tranzorbs after
the trad name.

The way they work is they have a "threshold" voltage, which you
select to be just above the normal working voltage.  Below the
threshold voltage, a VDR is an open circuit.  Go above the
threshold, and the resistance drops to a very low value.

A surge protected "thing" tyhpically has three VDRs, one each
between L-N, L-E and N-E.

So, if a surge comes along, a surge being a very short duration high
voltage condition, the VDR just puts a short across the lines, and
thus dissapates the surge.  Now a short across the mains isnt funny,
and it is only because of the very short duration that the VDR can
survive.  If the overvoltage were to persist, the VDR explodes in
short order.




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