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Surge Suppressors, Circuit Breakers, and Red Herrings

September 20th, 2010 Comments off

A large municipality had a very frightening situation recently.  It seems that a dozen surge suppressor strips (with internal circuit breakers) underneath office workers’ desks had overheated to the point of starting small fires internal to the strips.

 

MIDWEST was contracted to solve the problem.  MIDWEST’s Engineering Department installed Dranetz line monitoring equipment (Dranetz is the gold standard of line monitors). 

 

The results revealed a very, very interesting situation.  The line monitoring equipment discovered two anomalies. 

 

First, the 120 volt line was continuously about 8% too high;utilities guarantee a high line tolerance of 5%.  Thus the customer’s line voltage was 3% higher than the utility allows.

 

Secondly, some large short term voltages were measured on the customer’s neutral to ground by the Dranetz.   

 

In addition, forensic analysis was performed on the surge suppressors.  It was found that in all instances, the internal voltage spike protecting component, the metal oxide varistors (MOV) had burned themselves up, due to long term overheating.  In doing so, much of the underlying printed circuit board was also burned up and carbonized.  This carbonized material continued to support a small current and generate heating in the strips.  However, the internal circuit breakers never tripped.  The fact that the circuit breakers never opened is a very critical clue.

 

At first glance, the “obvious” conclusion one could draw is that the 8% high line condition caused the varistors to conduct, causing the overheating.   

 

But, the obvious conclusion would be wrong.  The 8% high line condition is just a red herring in this investigation.  Referring to the V-I characteristic curves of the varistors, an 8% overvoltage would cause negligible current flow.  In order for the 1 watt rating of the varistor to be exceeded, double or triple the nominal voltage would have to be applied for at least several minutes; the varistors would conduct around an ampere or less.   This is far less than the 15 ampere rated current of the circuit breaker.   

 

The surge suppressor strips’ melted plastic housings, and the extensive baking of the printed circuit boards to carbonization indicated heating on the order of minutes.  

 

On the other hand, a typical large voltage spike of several thousand volts tens lasting tens of microseconds tend to explode varistors, creating a plasma ball, and spraying superheated varistor material and copper everywhere around the sacrificial varistor.   The plasma ball would act like a dead short circuit, causing an internal arc flash of hundreds of amperes.  This would have tripped the circuit breakers.  A typical varistor explosion usually sounds like a gunshot; however, no such noises were heard by personnel.  The author has personally witnessed these explosions during destructive testing of varistors under controlled conditions.  The surge suppressor post-mortems’ also revealed no deposited spray of bulk varistor material or of copper.

 

The only conclusion that could be drawn is that double or triple the nominal line voltage actually was existent for several minutes, rather than a one-time voltage spike.  This conclusion is supported by the anomalous Dranetz neutral to ground measurements.