over voltage protection

[Doc]

http://users.telenet.be/worldstandards/electricity.htm#3voltage

[andyf97]

I dont know what standards are outside europe but most services we have used in events within europe for 20 years have had 380v ac especially from portable generators.

Heroic, What is a MOV?

I was thinking there must be a sollution that can rapid off the power if the voltage goes to a certain level without blowig fuses. Like a relay that will stay closed unless the voltge required to energize its coil is reached.

So lets say the relay with normaly closed contacts needs 260vac to operate, in normal mode no power the relay contacts will be made, if the coil goes active we are over voltage and the relays open up breaking the power to the lasers. That could be extended to sound an alarm or trigger a power off to other devices.

[heroic]

Apparently 380V was old 3 phase?

And yes, MOV's are designed to do just this. Although usually they sacrifice themselves by conducting current, which in turn blows the fuse in your laser to stop the power. You have to replace the MOV every time it does this however.

They won't sacrifice themselves if you use a fast trip breaker and an appropriately sized (read: rated for several times the fault current rating of the breaker) MOV.

MOVs are metal oxide varistors. They are basically threshold devices that switch from nonconductive to conductive when a sharply defined voltage is exceeded. They then continue to conduct until voltage is removed (ie., the breaker trips). Therefore the rating you need is defined by the maximum current your source can provide multiplied by the response time of your breaker.

[andyf97]

They won't sacrifice themselves if you use a fast trip breaker and an appropriately sized (read: rated for several times the fault current rating of the breaker) MOV.

MOVs are metal oxide varistors. They are basically threshold devices that switch from nonconductive to conductive when a sharply defined voltage is exceeded. They then continue to conduct until voltage is removed (ie., the breaker trips). Therefore the rating you need is defined by the maximum current your source can provide multiplied by the response time of your breaker.

So the idea with the MOV is to short the power to trip the RCB before the laser fuse blows?

[Doc]

If the line plug is correctly fused; that will blow first.

I use MOVs in conjunction with inductor/resistor limiting to protect the NST on my tesla coil from extremely high voltage spikes, I didn't realise they could clamp within such a small window, but as they say; you learn something new every day (especially on this forum!).

[heroic]

So the idea with the MOV is to short the power to trip the RCB before the laser fuse blows?

Yes, if you use a fast MCB rather than an RCB it can work.

[Doc]

I kind of doubt that a 30A breaker (most likely motor rated on a 380v, 3 phase supply) will trip before a 5A fuse lets go.

[heroic]

I kind of doubt that a 30A breaker (most likely motor rated on a 380v, 3 phase supply) will trip before a 5A fuse lets go.

Uh, no, you misunderstand. You do not rely on the external breaker. You add an extra panel mount magnetic breaker inside the projector. It is rated at just above the projector's peak power dissipation. The MOVs are attached to the output of the breaker. This way, the extra breaker will trip as soon as incorrect power is applied.

[Doc]

Ok, so taking the ideal MOV for the job a 524V25, rated at 240v max continuous; at 390v it will be conducting 1mA, at 690v it will be conducting 5A.

[heroic]

Ok, so taking the ideal MOV for the job a 524V25, rated at 240v max continuous; at 390v it will be conducting 1mA, at 690v it will be conducting 5A.

Remember that the peak voltage of 240v RMS is a lot higher than 240v. These parts are rated for 390v DC. Applying higher voltages than 240v *will* trip an MCB. (I've seen it). I don't know where you're getting the 5A figure from but it doesn't accord with my experience of MOV conduction. These parts rapidly drop to very low resistance once conduction begins.