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?
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Very good answer by Robert Baer.
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There is an often repeated idea that a MOV shunts current to ground
and that protects the device.
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A MOV across a relay coil shunts nothing to ground and protects entirely by voltage limitation, absorbing the energy stored in the relay coil.

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An airplane protects equipment from lightning entirely by voltage limitation and not at all by earthing.

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For a service panel suppressor, with a MOV from hot to neutral/ground/earthing-electrode-(all connected in US services), say a 10,000A surge arrives on a hot service wire. The MOV acts to clamp the voltage from hot to neutral/ground/electrode. That results in almost all the surge being dumped to earth. The MOV keeps the voltage from hot to neutral/system-ground at a survivable voltage for equipment. (At 10,000A it may not be safe for all connected equipment, but for most equipment.) Because of finite resistance to earth, the ground potential of the hot–neutral–ground will rise thousands of volts above 'absolute' ground potential. The MOV absorbs a very small fraction of the incoming surge energy.
Was the protection the voltage limitation (H-N-G)or earthing the surge? Both.

(Another critical element of protection in the example above is keeping the ground reference of power and cable and phone at the same potential, even though it is thousands of volts above 'absolute' ground potential. That requires a *short* wire from phone/cable entry protectors to the ground at the power service.)

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For a plug-in suppressor, the impedance of the branch circuit 'ground' wire prevents much earthing of a surge. But the impedance of the branch circuit also prevents much surge current, and thus surge energy from reaching the suppressor. The suppressor clamps the voltage on all wires (power and signal) to the common ground at the suppressor. The voltage between wires going to the protected equipment is safe for the protected equipment.
Is the protection the voltage limitation (H-N-G) or earthing the surge? Almost entirely from voltage limitation. (See the example in the IEEE guide starting pdf page 40.)
<http://www.mikeholt.com/files/PDF/LightningGuide_FINALpublishedversion_May051.pdf>

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What happens when there is a plug-in suppressor and no service panel suppressor?
At about 6,000V (US) there is arc-over from hot bus to panel/ground/neutral/electrode. After the arc is established the voltage across the arc falls to hundreds of volts. That dumps most of the surge energy to earth. Again, because of branch circuit impedance, not much surge energy can reach the suppressor. [There is also arc-over at receptacles at about 6,000V (US)].
In experiments with a MOV at the end of branch circuits 30 ft and longer, with a surge current up to 10,000A, the maximum MOV energy dissipation was 35J. In 13 of 15 cases it was 1 Joule or less.
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Of course the current might be extreme
since the move acts like a short
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As others have said, MOVs don’t short, they clamp. For power circuits, there will probably always be over 300V (US) across the MOV. High surge currents drive the clamp voltage up significantly.

It is very unlikely there will be a surge current on a hot service wire over 10,000A. Service panel suppressors with much higher ratings are readily available.
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The
transient high voltage goes away whether current is shunted or not,
doesn't it?
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A surge from lightning will almost always have multiple paths to earth.

--
bud--



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