Surge protectors in outlet strips have been around for at least 35 years. Yet equipment still fails. At my church, the Panasonic video camera control unit (CU)was dead one Sunday morning.
During the investigation, I uncovered an indication of what happened. Inside the CU nothing was found burned or otherwise damaged. The unit wouldn’t turn on. I check the power transformer and found the 120 volt input coil was open. The primary wiring in a low wattage 120v/24v transformer is very small, usually 28ga enamel wire. It doesn’t take much current to burn it into.
I found the CU plugged into an older surge protecting outlet strip. It was an expensive one but dated 2005, roughly 8 years old. It had a small red light that would indicate a surge protector failure, but it wasn’t on. Inside it, I found some red ¾” disks that had exploded. I checked with the church maintenance man, there were lightning storms that week, but no close strikes that week he knew of.
I had found the surge protector in the on position and the control unit switch was left on.
The outlet strip protection failed but who knows when? It is usually difficult for a surge to pass through a transformer due to its inductive characteristics at that frequency. The transformer, no doubt, had been subjected to many surges, the last one finally weakened the primary turn’s wire, and it opened.
Any outlet strip less than $30, like most, uses a component called a MOV or Metal Oxide Varistor. They are rated with their heat dissipation characteristics measured in joules. A MOV is built by compressing a recipe of zinc oxide fragments and binders into ½” – ¾” disks. The result is a variable resistor—as the voltage increases across its terminals, its resistance in this case, decreases. When a voltage is applied higher than its design voltage, the MOV starts to conduct drawing away damaging surge currents.
Some MOV’s are made in a way that if it fails with the material shorted, a bypass circuit lights a small red light shines indicating a failure. If the MOV blows out everything, unfortunately the light doesn’t work. MOV’s usually short with many small discharges, but explode with large currents.
There are two advantages and three problems with MOV’s.
Their advantages are low cost and large surge currents. Their disadvantages are they are slower to respond to typical lightning wave forms, they degrade over time as they are used, they do not clamp voltage as you would expect, and are misapplied with misleading labeling by manufacturers.
First, lightning wave forms are fast. A pulse can be 1.2 microseconds in duration. A typical MOV responds is 8 microseconds. The equipment components are stressed during the 7 microseconds the MOV starts to work. Better surge protectors use surge diodes.
Second, MOV’s degrade with use. As a MOV conducts surge currents, the zinc particles tend to fuse loosing their resistive characteristics. Alss currents increase their life decreases even faster. The range is significant; a MOV may last for years conducting small 50 amp surges or last one day with a 3000 amp surge. The user has no protection or knows it unless it shorts and turns on the little red light.
Third, as the MOV conducts increasing currents, the voltage across its terminals increases. There is no voltage clamping like a surge diode offers. The equipment you are protecting experiences higher than normal voltages. True, the MOV is conducting most of the problem currents, but some damaging voltages and currents are getting through.
Lastly, manufacturers install MOV’s in a parallel manner, basically stacking them up with the hope that the combination will have one MOV acting on a surge for protection. But, they advertise their equipment offers joule ratings to unbelievable values; they claim this with their stacking MOV’s. By installing six MOV’s each at 300 joules and 1000 amps, the outlet protector now advertises as an 1800 joule, 6000 amp protector. Being in parallel you would think this is true except for one problem. All MOV’s are not manufactured with precision values. At the time of a surge, and you have six MOV’s. Each will not conduct evenly. One will conduct more; probably will fail; now you have five. Another high surge, another fails, you have four, then eventually one, and then none.
Do you now trust an outlet strip? Probably not; do like the manufacturers recommend: replace your outlet strip yearly or after a hot lightning season.