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240 V AC is the RMS.
The tolerance of mains is +-10 %, so the max. AC voltage is 240 V × 1.1 = 264 V.
The peak is 264 × sqrt(2) = 374 V.
Your varistor my not become active at or below this voltage as this is normal operating condition.
Typically Varistors have an AC rating that takes these factors into account. A varistor with an AC rating of 264 V or higher is suitable. Often 275 V varistors are used to leave some headroom for voltage spikes above + 10 %.
The tolerance of mains is +-10 %, so the max. AC voltage is 240 V × 1.1 = 264 V.
The peak is 264 × sqrt(2) = 374 V.
Your varistor my not become active at or below this voltage as this is normal operating condition.
Typically Varistors have an AC rating that takes these factors into account. A varistor with an AC rating of 264 V or higher is suitable. Often 275 V varistors are used to leave some headroom for voltage spikes above + 10 %.
Thanks @Harald Kapp. I guess under normal operation, peak voltages are only hit very occasionally and for a short period of time which isn't enough to cause the Varistor to go low resistance and blow the incoming fuse?
That is the idea of the varistor in that place. Only under overload conditions should the varistor become conducting and thus trigger the fuse. That the varistor itself dies during this act is an unfortunate side effect.
However. note that this failure is not necessarily the result of a single overload event. Depending on the quality of your mains supply the varistor may experience multiple small overload events during its lifetime. Every single event not being strong enough to trigger the fuse or destroy the varistor. But with every single small event the varistor dies bit for bit until it finally blows up in a single catastrophic failure.
If such a failure happens rather frequently in your home, consider putting a powerline filter between the appliance and the mains outlet.
However. note that this failure is not necessarily the result of a single overload event. Depending on the quality of your mains supply the varistor may experience multiple small overload events during its lifetime. Every single event not being strong enough to trigger the fuse or destroy the varistor. But with every single small event the varistor dies bit for bit until it finally blows up in a single catastrophic failure.
If such a failure happens rather frequently in your home, consider putting a powerline filter between the appliance and the mains outlet.
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