Blown MOV's

[RSEC]

I have a relay in a circuit with a MOV (V130LA10AP) across the contacts and it is fused (5 Amp Fast Blow). I am switching 120 VAC to the customer’s equipment. Everything is fine until the electrician messes up and gets things out of phase and then there is 240 VAC in my circuit. Then the MOV blows and not the fuse. I want to reduce the size of the fuse so it blows and not the MOV Anyone have a suggestion on what size fuse to use or will the MOV always blow before the fuse?

Thank You, Russ

 

[Martaine2005]

Are you converting the unit from 240V to 120V? Or have you designed a remote circuit that powers the unit?.
Can you elaborate please.

Martin

 

[RSEC]

I am not converting anything.

My problem is electricians that don't check the phasing. If the installing electrician doesn't check the phasing or like at the last one he got some wires mixed up and blew out 3 of 7 relay boards. I could repair the boards but it would be easier to just replace a fuse.

I have a separate circuit that powers a 12vdc relay. My problem is the voltage across the contacts. I use the MOV's across the contacts to prevent voltage spikes in the equipment that is downstream, and that could be another relay coil, solenoid valve or some other piece of electronic equipment. Lately I have been using these systems in the car rental areas of airport parking garages, and, so that they are never out of business two of these systems are used in parallel so if one fails there is still another for backup. When they are up and running everything is fine.

My website is https://rselec.com/

 

[Harald Kapp]

I want to reduce the size of the fuse so it blows and not the MOV

That will probably not work well. The fuse blows due to overcurrent. The current is a consequence of the MOV becoming conductive due to ovevoltage. So it will always be the MOV that acts first, then the fuse. The fuse is there only to prevent fire that may result from the MOV being stressed by high power when it has been triggered and the trigger condition (overvoltage) persists longer than the MOV can take (MOVs are meant to suppress short spikes only).
You might try a faster fuse with a lower trigger current, but then you risk the fuse being triggered by even a short overvoltage event and then the MOV will be useless afterwards as the fuse is open.

My problem is electricians that don't check the phasing.

Here's the real problem: sloppy workmanship. A MOV is the wrong tool to correct this.

I use the MOV's across the contacts to prevent voltage spikes in the equipment that is downstream

How is that supposed to work? Imho not at all:

• contact open: the full voltage will be across the open contact, so will overvoltage be. The equipment downstream is unaffected by the overvoltage, the relay takes the full blow.
• contact closed: the MOV is short circuited and the full voltage is present at the downstream equipment, including overvoltage.

If you want to protect the downstream equipment from overvoltage, the MOV should be across the equipment's power port, the fuse in series:

Here the MOV limits overvoltage and in case of excess current the fuse opens and takes the equipment off the power.

 

[RSEC]

This is my output circuit

I did some catastrophic fuse testing starting with a 1/2 amp and worked my way up. 4 amp will blow and not damage anything but a 5 amp will destroy things. I was using 5 amp fuses I'll do a little more testing with 4's and I think I will go to 4's. The reason the fuse is on the Load side and not the line side is because it was an after thought and it was the only place it would fit on the board.

Thank You for your help, Russ

 

[Harald Kapp]

Please clarify: There's one line connection and one load connection. How is that supposed to create a closed circuit for current?
Do you mean line as "input" (phase from mains) and load as "output" (to load and from the load back to neutral via a connection we do not see in this diagram?
As I tried to explain in my post #4: your circuit will not protect the load from overvoltage:

• With the relay contacts open, any voltage will drop across the open contacts, the load will not be affected by it. On the contrary: With wrong wiring and 240 V instead of 120 V and thus he overvoltage being higher than the MOVs max. voltage, the MOV will start to conduct and actually allow the overvoltage to reach the load. The load will then draw too much current. This is why the fuse triggers.
• With the relay contacts closed, the MOV is short circuited and without protective function.

 

[Tha fios agaibh]

Everything is fine until the electrician messes up and gets things out of phase

This is the real problem here. Unfortunately not everything can be idiot proof.

I agree with Harald. Place the MOV across the load.
You can add a snubber across the relay contacts if you want, but a MOV won't do anything.

 

[RSEC]

I hope this answers your question. This is a simplified drawing of what we are doing.


Switch C is normally open. Both Motors A & B do the same thing, pump fuel to the same place and they can operate one at a time or together. If motor A fails then it can be controlled by the motor B controls after closing switch C or A can control motor B. If the electrician does not keep the Line (L1 - 120 VAC) on each system on the same phase things start to blow.

These are not the easiest things to wire because these control circuits are running up and down through 4 different levels in these parking garages and this is only a small part of it.

 

[Harald Kapp]

The MOV is still in the wrong place. This is how it needs to be:

 

[Tha fios agaibh]

Problem is if one side hooks to L1 and the other to L2 there's a dead short if both sides activate while sw C is on. If both are L1 or L2 there's no problem.

I would just isolate the two sides (split phase) with either a rib relay or tap into the auto relay coil directly.
That way it can be hooked up to either L1 or L2 without shorting.
Notice I put the lollipops (MOVs) directly across the coils.

 

[Harald Kapp]

These are not the easiest things to wire because these control circuits are running up and down through 4 different levels in these parking garages and this is only a small part of it.

Correct wiring is the A and O of such a setup.
MOVs are not meant to protect against miswiring. They are meant to protect from transient overvoltages (not permanent ones).

 

[Haberkern]

Someone may already have mentioned this but a 130V MOV or TVS is the peak to peal rating - not the RMs. You typically want a 180V to 220V TVS for 120VAC(rms) to keep it from popping. Double if you have to seal with out of phase.

 

[Harald Kapp]

130V MOV or TVS is the peak to peal rating - not the RMs.

I disagree. That in not generally true.

Have a look at this excerpt from a MOV datasheet:

The rating AC 75 V (RMS) is the continuous rating, the continuous TMS voltage the MOV can withstand without breakdown. The corresponding DC value is 100 V, roughly sqrt(s) × 75 V (a tad less).
The varistor (MOV) voltage at 1 mA is 120 V (up to 200 V at higher currents).

A comparable 75 V TVS si defined in a different way, see this excerpt:

We have a standoff voltage (no breakdown at or below this voltage) of 75 V and breakdown between 77.8 V and 86.4 V. Here the voltages are peak values, not RMS.