MOV blown on NiMH 4xAA cell charger - cause?

[FuZZ1L0G1C]

Hi.
My NiMH 4xAA cell charger is manufactured by "Duracell", the rechargeable batteries I bought are "Energizer recharge" 2300mAh each, ACCU NH15 1.2V NiMH - HR6.
I have started using rechargeable cells with this recently bought charger as my power-hungry DSLR Pentax eats alkalines at an alarming rate.
Over the weekend, I had used the rechargeable batteries, so was recharging them over several hours, checking every so often that cells and charger weren't getting too hot.
The charge started with LED's red, as cells were depleted, then later turned to green.
To give them an extra boost, I left them for longer in the charger.
Later, a loud bang from charger, both LED's gone off.
Unplugged and checked cells - all 4 OK.
Opened charger and found mains fuse is OK, no blown caps or resistors, but MOV at mains input reads open circuit (OL) on ohms scale.
Shouldn't a good MOV be a low reading, like the fuse?
I plan to get another charger, and/or or replace the MOV, but first:
What could cause the MOV to blow -
too long on charge?
wrong cells (charger specifies to use NiMH)?
could the 2300mAh capacity rating be too heavy for the charger (then surely the secondary cct would have gone haywire - not primary input)?

 

[(*steve*)]

A good MOV should read as a high resistance (or open). Think of them like a zener for AC.

A failed MOV typically looks damaged (maybe even blown apart).

 

[FuZZ1L0G1C]

A good MOV should read as a high resistance (or open). Think of them like a zener for AC.

A failed MOV typically looks damaged (maybe even blown apart).

Thank you for that info..
As you may have guessed by now, I know zilch about MOV's.
.. Just realized.. the MOV is directly across the supply (after the fuse), so it wouldn't be low R as I thought.
(Note to self - look up / Google "MOV").
ADD:
I now think I found the source of the bang - the processor on the board's underside is discolored, bluish/grey patterns as if heated, and barely visible scorch peeping out from on end on pcb under this IC.
Smells slightly of burnt paxolin.
Looks as if IC popped its bottom end.
Will scrap, strip for exam, and buy a new charger.

 

[Bluejets]

Maybe the back door slammed shut and knocked the plug out of the wall socket.

 

[FuZZ1L0G1C]

Maybe the back door slammed shut and knocked the plug out of the wall socket.

Or faulty IC or load-shedding spikes and surges.
The socket is on a 5-way floor-standing horizontal multi-plug, so it could have been an earthquake..