01-10-2015, 10:57 AM
Apologies for this, but it's just come up in the day job.
Is it permissable to connect a metal oxide varistor bewteen live and earth (ie safety earth) for RFI suppression?
Sure, the varistor may clip spikes and shunt them to earth. But varistors gradually become leaky over time, as they absorb the energy of spike after spike.
Capacitors connected in such a position have to be designed to fail open-circuit ('Y' rated parts), so that a failure doesn't cause a large earth current, and putting significant potential on the earth line (and so making metalwork of all other equipment connected to the same earth point, similarly semi-live).
The thought of a varistor being connected in such a position worries me, and I can not recall seeing it done in practice.
Is it permissable to connect a metal oxide varistor bewteen live and earth (ie safety earth) for RFI suppression?
Sure, the varistor may clip spikes and shunt them to earth. But varistors gradually become leaky over time, as they absorb the energy of spike after spike.
Capacitors connected in such a position have to be designed to fail open-circuit ('Y' rated parts), so that a failure doesn't cause a large earth current, and putting significant potential on the earth line (and so making metalwork of all other equipment connected to the same earth point, similarly semi-live).
The thought of a varistor being connected in such a position worries me, and I can not recall seeing it done in practice.
