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Why didn’t the Circuit Breaker trip in a Surge Suppressor Strip?

August 12th, 2010

MIDWEST received a call for help lately when a client had a dozen surge suppressor strips (with circuit breakers) virtually start on fire.  The interesting thing about this client was that it was a large metropolitan area’s city hall.  The surge suppressor strips were powering the computers that controlled the jail.  The surge suppressor strips had been in place for 10 years without incident.  All of a sudden, on two different floors, a dozen surge suppressor strips overheated.  The strange part is that the circuit breakers inside them did not trip.  Forensic examination  by MIDWEST revealed that the metal oxide varistors inside had overheated, causing the varistors and printed circuit board to burn up;  these were essentially carbonized.  This carbonized material supported arcing, and generated great quantities of heat.  But why didn’t the internal circuit breaker or any building circuit breaker trip when the arcs occurred?


First, a circuit breaker is an electromechanical device that interrupts a circuit when a large current flows through it.  But it has to be a large current, like three to ten times rated current, depending on the particular circuit breaker’s curves.  This is the breaker’s trip current; in reality, there are engineering curves and graphs that define a circuit breaker’s exact trip.  For a better description, see “How Circuit Breakers Work” and Wikipedia’s entry on circuit breakers.


Why didn’t the circuit breakers trip?  The answer is that if the resistance of the carbon tracking is high enough to limit the current flow to less than the trip current, then the breaker won’t trip.  And the heat just keeps building up inside the strip, resulting in fire.  Thus circuit breakers are not a cure-all; in general, a circuit breaker will trip when presented with a high current short circuit.  But if the fault impedance is current limiting, the circuit breaker gives no protection.  This current limited situation is the cause of most electrical fires and is highly dangerous.  These can be very elusive. 


But, one great solution to the problem is offered by MIDWEST’S Infrared Thermography Service.  But that is the subject of another blog.


  1. August 18th, 2010 at 14:45 | #1

    Surge suppressor strips 10 years old should have been UL1449 v2 listed. The technology available for surge suppressors sold new ten years ago most likely included internal thernal devices that should have cut-off the internal power to the strip. Even if the circuit breaker did not trip there should have not been enough heat to ignite a fire.

    The surge suppressor strips had been in place for 10 years without incident, but, when were these strips manufactured?

  2. September 15th, 2010 at 22:04 | #2

    The surge suppressor strips did not contain a thermal disconnect. They do not list either the date of manufacture, nor the version of UL 1449 to which they comply. The 10 year figure was given by a site electrician. I’m not sure how accurate the figure is, but he said they were all bought together, and were expensive at the time.

    A recent trip to Walmart revealed that not one single brand of surge suppressor strip listed either a UL 1449 revision, a date of manufacture, nor any such indication that they contained any failsafe components such as a thermal disconnect. So, it would seem that the only way to tell is to buy a strip, and tear it apart to find out.

    In general, I would recommend buying a high quality strip with the highest Joule rating offered. For instance, Belkin, a major name brand, offers their top of the line strips with over a 4300 Joule rating and a $300,000 Connected Equipment Warranty.

    It does not pay to be cheap when buying a surge suppressor. They are very cheap insurance against catastrophe. Part of the reason to buy a high Joule rating, and hence an expensive strip, is simply the market place reality that you usually get what you pay for (or, equivalently, if you buy cheap, you get a cheap manufacture).

    As far as the heat generated, if the applied line voltage during the high line condition was just 150 Vrms, and if just 1 ampere of current flowed through the carbonized material, this would generate 150 watts of heat. If the applied voltage was 250 Vrms, and if 10 amperes flowed, then this would generate 2500 watts. Neither current would be enough to trip any 15 Ampere circuit breaker. However, either scenario is more than enough heat to melt and cook the innards, and eventually start a fire.

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