Postmortem on two power strips after a surge

During this weekend, I came home to a dark apartment. This is normal, as the sun usually sets in the afternoon leaving things rather dark. But this time, I was unable to turn the light on. The breaker box was consulted, and one breaker (only two 20 amps in this tiny apartment, talk about low end wiring...) was off. I hit it several times, but it would not stay on. Unplugging everything including the fridge solved the problem. Adding items back incrementally (like the Apollo 13 movie) let me figure out that the microwave was probably to blame. At this stage, the working theory was that the microwave had burnt out finally after all these years.

However, upon plugging it in, I found that everything was fine! So the power strip used to hook up the microwave must have been the problem. The strip did indeed have an odd smell to it, like burning electronics. Things seemed to be fine after this, and I settled in for some episodes of the West Wing. However my UPS kept clicking on and off, and soon the lights started to brighten and dim back and forth. At times they got incredibly bright, indicating a powerful over-volt power surge. I was not able to get my multi tester out during the process (I was more worried about the safety of my computers and TV) but a safe bet would be 150 volts, perhaps more. The incandescent bulb in the bathroom got so bright at one point that I expected it to blow up at any moment (or at least blow out, but it never did!)

Long story short, the apartment electronics are getting totally upgraded during this week, with power outages during the day. Nothing was damaged but two power strips, which is a miracle.

I decided to dissect the strips to find out was was going on inside. Here are the results:




I found it interesting that the fuses did not burn out, but the first items to fail were the metal oxide varistors. These (blue) items are semiconductors that conduct varying amounts of electricity depending on voltage:


If voltage becomes high, these items "open up" allowing current to shunt surges away from the devices that one wants to protect. Obviously these overheated and burnt. One would expect the system is designed to fail this way, resulting in some local burning but no major fire hazard and no damage to my microwave or alarm clock.

"The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.
For example, follow-through current as a result of a lightning strike may generate excessive current that permanently damages a varistor. In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress.
Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike, as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.
A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged."

- Varistor

It was interesting to see that one strip burnt out without causing any major problems after; it did not trip the breaker and only buzzed a bit. The other was fully shorted out and was able to draw enough to trip a 20 amp breaker.

This is the model found in one of the power strips. Can anyone help me understand the ratings? 1 watt sounds like the normal operation draw, no? And 130 volts AC sounds like the threshold to start shunting? Certainly that can't be the failure point... And energy (2ms)J what exactly does that mean?