[TowerTalk] Fwd: WHOLE HOUSE SURGE SUPRESSORS

[Ian White GM3SEK]

David Robbins wrote:
>the most important function of a whole house or similar large arrester 
>serving a building is to equalize the voltage between all the 
>conductors so that all the equipment connected to it is as the same 
>potential.

We don't truly mean "equalize" and "same potential" here.

When MOVs conduct, they do not crowbar their two terminals together to 
equalize the voltages. They act more like bidirectional zener diodes, so 
they attempt to limit the applied voltage *difference*... which can 
still be quite large.

In a typical US home installation, the threshold voltage of the MOVs 
must be well above 115V AC to ensure that the MOVs will not conduct at 
the peaks of normal line voltage. When a surge arrives and the MOVs do 
begin to conduct, the clamping action is quite 'soft' so the voltage 
difference between the two lines can easily exceed 200V (depending on 
the source impedance from which the surge arrives).

When seen against the grand scale of lightning voltages, 200V might 
conceivably be called "the same potential". But the connected equipment 
doesn't share that view - it still sees the 200V across its terminals.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek