Spark gaps versus Varistors 1

[chiefwvfc]

I am curious if any one call tell me the advantages or disadvantages of using a spark gap over a varistor for over voltage protection.

We are using a spark gap to keep an output voltage from going over 400Vrms.

 

[RalphChristie]

With the same nominal protection level, the MOV (Metal-oxide varistor) gives lower overvoltages and consequently lower risk of failure than the spark gap, mainly because it conducts a rather high current at voltages far below the nominal protection level. The lack of sparkover transient is also a benefit. Spark gaps give the highest overvoltage and overinsulation seems necessary.


Regards

Ralph

 

[buzzp]

Varistors also breakdown with each overvoltage situation, reducing its capacity next time.

 

[tinfoil]

The performance of a spark gap can vary depending on local environmental conditions, whereas a varistor will give more consistant results (as long as they are not overstressed to the point of a breakdown)

 

[RalphChristie]

The only advantage I can think of spark gaps is the fact that they are cheaper than varistors. (Short term)

 

[sreid]

Generally speaking spark gaps will absorb higher energy than MOVs. But MOVs conduct faster and have a tighter range of conduct voltages. TVS are faster yet but absorb less energy. Some times a combination of devices (buffered with inductance) are used

 

[busbar]

 
400V seems quite low for repeatable triggering of spark gaps, and may be affected by routine change in atmospheric conditions.

 

[jghrist]

See

http://www.cci-msc.com/Surgecontrol/fundamentals.html

for filtering and surge suppression fundamentals. The link at the bottom is to a .pdf file that has detailed explanations of different surge suppression devices.

 

[mc5w]

2 reasons why spark gaps are no longer used:

1. In order to flashover, the voltage rise time must be very steep and is in fact a higher rise time that requires derating of insulation.

2. A spark gap that is the minimum amount that can be used will continue to arc after the lightning strike until the power is deenergized. This is one of the reasons why insulators flash over and the power has to be turned off and reclosed to stop a continuing arcing fault.

On their 7,620Y13,200 volt overhead distribution lines Florida Power and Light either uses a 250% insulation level with lightning arrestors every 3 pole OR and overhead neutral. Usually the overhead neutral is used only in rural areas where there is no chance that some of the arrestors can be protecting a transformer anyways like in urban areas. The reason why is that Florida has heavy duty lightning that requires heavy duty wiring.

Mike Cole, mc5w@earthlink.net