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Wednesday, April 7, 2010

A Surge protector that works!!!

I returned home to find that a fire had damaged my workshop while I was away. A power surge of over 1000 volts was caused by a falling 60kv transmission line. This happens all the time where I live because PG & E has constructed one of the worst transmission and distribution systems of anywhere in the US. I used to loose from $200 to $10,000 worth of line connected equipment every winter, until I discovered a surge protector that actually works.
Virtually all surge protectors that are affordable to consumers, who are not wealthy, contain metal oxide varistors (MOVs) as their primary protective device. MOVs are very useful but they are far from perfect for protection against surges. They absorb energy in a surge by converting it to heat. When the voltage across which they are connected, rises above a certain design level, they become less resistive and act like a clamp. The more MOVs and the higher the rating (in joules of energy) the greater the protection. The intensity and duration of the surge determine whether or not the protection will be adequate. MOVs are great for short duration spikes even at very high voltages. They act quickly and offer for many common scenarios. They are not good for protection against a long duration high voltage. They are simply destroyed and so is the connected load. There are other problems with MOVs of which most people are unaware.
MOVs are damaged by “stress”. Their ability to absorb energy continually decreases as they are subjected to high voltages, over time. There is no way to determine the remaining capacity other than by destructive testing. Visual appearance will indicate a MOV that is destroyed and there are measurements that can be made to indicate if they are “functioning” but there is no way to know what capacity remains. Therefore the only way to assure protection is to replace them regularly. This can get expensive. Another little known fact is that MOVs can fail in a shorted condition. If they are connected again to the line, they can flare, creating a hot plasma that can start fires. If you have a power surge that trips the breaker or results in a power outage, you can have a fire once the power is restored. Better surge protectors have fuses or circuit breakers upstream of the MOVs to prevent a damaged MOV from being connected to the line. Many do not including the one that started a fire on my desk on year.
There is only one line of protectors of which I am aware, that works in a different and preferable way. Most people do not have the severe surges that I get, which are technically called “faults” because they are of long duration. However, the cost of these protectors is only slightly higher than the competition and their clever design also protects the MOVs in such a way that they are likely to last longer than those in other devices. Here is the secret:
Panamax makes a variety of surge protectors. Many of them are of the conventional design but one series has a feature called “protect or disconnect”. If you see this phrase on the package or the datasheet, this device is what I will describe. If it is not specifically stated, it is not likely to have this feature. The model M4-EX is one example of such a device. There are many other but even more that do not have this critical feature.
Protect of disconnect, works by sensing the line voltage and disconnecting the load from the line if voltage either too high or too low. Voltage level and time constants are built into the device are well chosen. The device rarely triggers falsely. The load is connected through an electromechanical relay which acts as a “deadman” switch. If there is any problem with the protective circuitry, the most likely result will be the disconnection of the load. Because the device also reacts to low voltage conditions, it protects against brownout caused damage as well.
The Panamax “protect or disconnect” surge protectors have several MOVs, but the majority are connected to the load side of the disconnect relay. This means that they are protected against surges as well and are less likely to suffer from stress induced capacity reduction.
It is amazing that Panamax makes these excellent devices are affordable prices. Most surge protectors offer little protection. They often include an insurance policy and you may actually collect but if you want your valuable equipment truly protected it is better to buy a quality device instead.
There is one scenario that the Panamax will not protect against. In five years, I have had only one failure to protect a device connected to a Panamax protector. This was due to a faulty design of the device, not the Panamax. A distribution line had become partially disconnected at a cross arm, creating what is called a “floater”. The line voltage continued to swing from low to high over a sustained period. I was sleeping and every time the power went on or off, I assumed it would be the last. I should have gotten up and shut off the main but I was unclear as to what was happening. The device that failed was a new refrigerator. The designer of this expensive piece of crap (Amana, but this is made by Maytag along with other brands such as Jenn Aire) did not bother to include a start delay in the design. This is a serious oversight. A start delay is a time delay relay that prevents the compressor from “short cycling” so that it does not start under a loaded condition. This was common practice in all refrigeration units until recently. New refrigerators do not use mechanical thermostats and defrost timers. They use microcontrollers and temperature sensors. This is done primarily to reduce cost. A circuit board that is the functional equivalent of two thermostats and a defrost cycle timer costs considerably less to manufacture and install. When these cheap boards fail (due to a power surge, for example) they are not cheap to replace. The manufactures sell them at highly inflated prices. The disturbing fact is that a start delay would cost nothing to add to such a control board. It would require a minor modification to the program residing in the microcontroller chip. I cannot imagine how a competent engineer could leave out such an important design factor. My guess is that the manufacturer prefers failures that can be blamed on power quality. They do not have to cover such damage under their warrantees and can sell expensive replacement parts. My refrigerator was replaced under warrantee but the appliance dealer probably had to call it a compressor failure and not mention power problems. Once I got the refrigerator home, the first thing I did was to install a start delay relay which cost me about $25.

1 comment:

  1. Thank you for providing the great information on a Surge protection Device . The pictorial representation is very helpful for every new user.