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What shunt mode surge protection?

by Nyal Mellor April 08, 2013

Shunt mode surge protectors are typically based on circuits that contain metal oxide varistor (MOVs) which, under normal operating voltages, act as an open circuit and allow no current to flow through them. The MOVs will start to conduct should the voltage level rise above the clamping voltage of the MOVs (i.e. a surge condition), shunting the potentially damaging surge away from connected equipment. MOVs are sacrificial devices – they only have capacity to absorb a certain number of surges before they fail. The main benefit of MOVs and the reason you see them used so much is that they are cheap.

The diagram below from SurgeX shows how an ‘all mode’ MOV device works.

The main issues with ‘all mode’ MOVs are as follows:

They operate by diverting surge energy to ground.
Voltage on the ground can rise to dangerously high levels in the event of a surge because the ground has a non-zero impedance resulting from the length of the electrical wiring between the outlet and the earthing point at the electrical panel. The worst risk is when devices are interconnected and some are on shunt mode surge suppressors and others not. This can create high voltages on the ground of the shunt mode protected device that then flow across the interconnects to devices not on the same surge protector, critically damaging input and output
circuitry.
They couple noise onto ground. MOVs have what is called leakage capacitance. From an electronic perspective leakage capacitance resembles a small value capacitor to ground. Since the impedance of a capacitor decreases with increasing frequency high frequency noise is coupled to equipment ground. This can cause noticeable hums and buzzes as well as degraded equipment performance, particularly with digital gear that uses ground as a reference point.

Single mode (hot to neutral) devices may be used, but we recommend using series mode technology instead. All mode protection can be used at the service panel in a whole house surge suppressor.

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Nyal Mellor

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