[TCML] MOV destruction

[David Speck]

Dennis,

MOVs degrade with time, and can often fail as a short circuit.  
Manufacturers recommend that you always install them in series with a 
fuse or circuit breaker.   I had an unprotected little one blow out in 
an alarm system that destroyed the 20 amp breaker is was attached to. 

As they do their protective thing, MOVs develop a lower and lower 
threshold until they are essentially a short circuit.  When the 
resistance of one MOV decreases, then the voltage drops across the 
others increase, accelerating their demise as well. 

Your 15 kV NST can produce ~21 kV peak to peak, but 16 MOVs in series 
should have been good to 28.8 kV, so it seems like you had enough MOVs, 
as long as they were good up their full nominal rating.  Were they new 
units or used when you started with them?  Did you have a lot of run 
time on them?

Dave

> I had 8 MOVs?(1800V ZNR) in series to ground on each leg of the transformer.? This essentially duplicated MOV arrays?that I have seen?others' using on their 15/60 coils.? My variac maxes out at 130v - also,?I didn't see any strikes to the primary or lower deck?at the time this occured.