Electronic – How to choose the right Metal Oxide Varistor (MOV)

surge-protectionvaristor

I would like to protect the input of a transformer against high voltage surges but I don't understand how I have to proceed to choose the right MOV despite what I read so far about them.

Below is how I tried to do to pick the right component.

The circuit

simulate this circuit – Schematic created using CircuitLab

The needs

I have a 115V / 60Hz power supply and I supply a load with 24VDC through a 24V/52VA transformer and the rights filtering capacitors. The value shown are the nominal ones but my system can work up to 160V(RMS) at the transformer input.

I would like to protect my system against high voltage surges (1000V max). I have often used TVS diodes in my previous circuits but the environment was different (surges value, dc…). As far as I understood, MOV are well fitted to protect system against voltage surges either in a AC or DC powered system. In fact I often see them at the input of transformers, that's why in this specific circuit I would like to use a MOV (plus I will learn how to use them).

So I would like, even when a 1000V surge appears, the voltage at the input of the transformer doesn't exceed 160V. Is it something achievable with a MOV ? From what I have read, yes. From what I have understood so far, no. Maybe this is not the right component to use.

What I understood

Just to be sure I am not in the wrong, a varistor is a voltage dependant resistor. Its electrical resistance decreases with the applied voltage. It has two functional operation modes : when the applied voltage is below its clamping voltage, the MOV is non-conductive (normal operation). Above, it becomes conductive and the voltage across it is limited to a value just above the clamping voltage value (from what I have read here).

Why I am stuck

Well, just because I don't understand. To me, the clamping voltage is the voltage value from which the MOV will become conductive and thus, the voltage across it will not exceed this value (or just a little, like a zener diode in fact). But then I read this post. This designer states that his system could work with 520V(RMS) max at the input. But in his calculus, he chooses a MOV with a 1500V clamping voltage. Does it mean that if ever in his system a voltage surge of 1200V appears, this spike will not be suppressed and the system will see it ? Or do I have to think differently and take in count the Maximum Allowable Voltage that seems to be the reverse stand off voltage according to Peter Smith in this post ? In this case, does it mean that the MOV starts clamping at 550V (which a little bit more than the 520V wanted by the OP) and the voltage accross the MOV can raise up to 1500V ? In this case, how a MOV can pretend to protect an equipment ?

If I use my case and think the same way as the OP's other post : The nominal voltage at the input in 115V(RMS) but can go up to 160V(RMS). If I use this datasheet, I guess I have to find a MOV with a maximum allowable voltage close to 160V(RMS). But what about the Clamping voltage max ?

I am missing something, I am missing how a MOV can protect an equipment against voltage surges. Can you help me ?

Thanks !