Re: Does a MOV shunt current or equalize voltage?

John Doe wrote:
From reading intelligent replies to w_tom, I have the impression
that a MOV effectively clamps the input and output together so that
there is no potential and therefore no current through the protected
device. Is that correct?

No. An MOV is a voltage dependent resistor that has a very high resistance over the normal range of voltages, but its resistance falls dramatically as the voltage rises above the normal range. This conduction helps limit how far above normal the line to line or line to ground voltage terminals of that point in the distribution system can swing by forming a voltage divider between the wiring impedance and the MOV resistance during the over voltage condition.

There is an often repeated idea that a MOV shunts current to ground
and that protects the device.

Unless the MOV arrangement has its own exclusive ground conductor system (available only on some breaker box mounted MOV systems) they have nothing to do with ground, directly. They limit voltage difference between nodes they are connected across by voltage divider action. As long as the connected devices can withstand the voltage within this clamped limit through all possible nodes connected to that device, the MOV has done its job. This may include more than just the line, neutral and ground receptacle terminals, like phone lines or cable TV lines. A useful MOV system should limit all node to node voltage to something below what the protected equipment can withstand. Keep in mind that during a severe surge, all those nodes at the protected equipment might have rather arbitrarily large voltage differences to other non connected nodes, including actual Earth potential.

Of course the current might be extreme
since the move acts like a short, but is that really what protects
the circuit? If shunting current is the protective action, doesn't
that mean the transient would have to be more than transient? The
transient high voltage goes away whether current is shunted or not,
doesn't it?

Or is it a combination of both?

The MOV is not a short, only a voltage dependent resistance. Assuming it survives the surge, it ceases to conduct significant current the moment the voltage difference across it falls below its rated operating voltage.

--
Regards,

John Popelish
.

Relevant Pages

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