Metal Oxide Varistors (movs)

[Khonwan]

I have in-line MOV’s on the power lines (3-ph + N) on my side of the meter that I’m responsible for. Once these operate to transfer the surge current, under a lightening strike, to ground via the neutral cable, must they then simply be disposed of? Or can they then (sometimes?) return to their normal pre-surge state (automatically or manually)?

Since 3 are in-line, If one goes, do all 3 go?

PEA were good enough to provide and install one set free this time (a goodwill gesture) but I’ve been quoted Bt450 per MOV locally (goods only). I am considering installing more.

Khonwan

[paul1970]

It depends on how much energy the MOV disspates during the lightining strike, if the surge current is within the SOA (Safe Operating Area) of the MOV it will reset itself and will continue to Function. If the amount of Joules dissapated goes above the SOA then the MOV may be permenantly damaged and would need replacing, usually visually inspecting a MOV should give you an idea as if there is a huge surge they will show evidence of overheating such as discolorasation or signs of burning or soemtimes just completely vaporising.

If in doubt it's best to replace them.

[Crossy]

Yep ^^^ A simple MOV doesn't really give much indication of its condition, a problem exacerbated by the fact that they tend to fail open-circuit leaving you with no protection

Basically, if its blasted into next week, you need a new one. If you've had a strike, you possibly need a new one. If you suspect a strike, you may need a new one

Since your installation has a history of being zapped I suggest that you may want to install something more high-tech in addition to the MOVs. Have a look at these chaps http://www.erico.com/products.asp?folderid=49 Erico have a Thai distributor so the kit should be reasonably easy to obtain.

These beasties don't come cheap, but they do have multiple protection systems (not just MOVs) and they have a life indicator to tell you when they're dead

[elkangorito]

I've just looked at the Erico site...it uses one of the brands that I have used extensively in the past...Critec. Critec have been around for years in Australia. They were a prefered government supplier before privatisation.

If you wish to go to their site, it is http://www.lightningman.com.au/surge_protection.html

Other good sites are Procel http://www.procel.com.au/ and http://www.novaris.com.au/

Even though these are Australian sites, the information available on them is very good. Remember that the Thai electrical system is very similar to that of the Australian electrical system.

Generally, for a standard domestic residence, a minimum 50kA at 8 to 20 microsecond protection device is recommended. Of course, the type of device you choose will depend upon the type of equipment (& its' value) you wish to protect.

Finally, good luck finding a Thai who is adept enough at;

1] choosing the correct device(s) for you, and

2] installing the device(s) correctly.

[Khonwan]

Thanks to all three of you. Having read your replies (and visited the links) I am inclined towards the status quo, i.e. doing nothing. The house and farm equipment have never been damaged (to date!) - only the kWh-meter and transformer surge-protectors. We have had a couple of serious electrical storms pass without incident since the meter was relocated away from the transformer (more importantly, away from the high voltage cables lightning ground) so perhaps my problems are resolved.

Thanks

Khonwan

[astral]

I guess this applies to the ones on the telephone line as well?

In the little black box on the wall outside.