Over voltage protection

The OVP clamps the input voltage to a range the device can safely handle. It is sized to absorb the spike safely, usually in joules energy, and peak current handling to maintain the clamp voltage. That is how a MOV works anyway, they are not usually used to blow fuses.

Hi, I am involved in the design of an equipment and we just left to the end some important details, wrong.

The equipment should have over voltage protection, ie, when a spike arrives it should be capable of keep working. Usually MOVs or SCRs are used in conjunction with fuses that blows when everything goes wrong, and that is acceptable, fuses are suppose to blow, the problem is that the OVP should not disconnect the equipment from the mains, that are the words of the engineer that certifies medical equipment. So how is suppose to work an OVP without disconnecting it from the mains?

If you have any idea, or answer please post it

Thank you, I really appreciate your help

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Use a polyswitch instead of a fuse then. It will "reset" once the fault is removed.

what we did with a few things at work that had issues blowing their MOV's and thus taking out the circuit was to feed the device via a transformer. The induction in the transformer solved many of the small glitches seen on the device. We still employed the same safety of MOV's and such on the secondary side in the event that the spike turned out to be more than just a short pulse.

In the case of medical equipment (Life saving type), is that suppose to be on a batter backup system?

Jamie..

"DonMack" wrote in news:itrcah$nnv$ snipped-for-privacy@dont-email.me:

MOVs shunt small spikes to ground and recover,and when shunting high energy spikes,also blow the fuse,and sometimes the MOVs blow apart from the excess energy. Blowing the fuse stops further energy from entering the device. Big surges may still jump the open fuse.Note the usual 3AG fuse is rated for 250 volts. Mainly,the surge energy has to be absorbed and dissipated,perferably without blowing things apart.

You should use a line filter(caps and inductors) before the MOV to slow down the surge risetime.MOVs are a "last-gasp" sort of protection,and they degrade with every surge shunted. Some surge protectors use a combination of inductor filters,MOVs,and gas discharge tubes. the GDTs handle far more energy without blowing apart than MOVs,but react a bit slower. For critical stuff,a UPS is the way to go.

You need to know what sort of "overvoltage protection" they specify. Peak voltage,peak energy,and duration.

--
Jim Yanik
jyanik
at
localnet
dot com

"DonMack" wrote in news:itrcah$nnv$ snipped-for-privacy@dont-email.me:

MOVs shunt small spikes to ground and recover,and when shunting high energy spikes,also blow the fuse,and sometimes the MOVs blow apart from the excess energy. Blowing the fuse stops further energy from entering the device. Big surges may still jump the open fuse.Note the usual 3AG fuse is rated for 250 volts. Mainly,the surge energy has to be absorbed and dissipated,perferably without blowing things apart.

You should use a line filter(caps and inductors) before the MOV to slow down the surge risetime.MOVs are a "last-gasp" sort of protection,and they degrade with every surge shunted. Some surge protectors use a combination of inductor filters,MOVs,and gas discharge tubes. the GDTs handle far more energy without blowing apart than MOVs,but react a bit slower. For critical stuff,a UPS is the way to go.

You need to know what sort of "overvoltage protection" they specify. Peak voltage,peak energy,and duration.

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What does any of this have to do with a polyswitch?

He is talking about using a crowbar but the fuse must be resettable. A polyswitch or resettable fuse will work. A polyswitch is self resetting.

By judicious choice of filter of the crowbar you can trip the polyswitch when there is a significant overvoltage for a significant time. Since this is transient behavior once the power returns to steady state the polyswitch will reset allow.

This is the easiest method of creating a resetting crowbar circuit. It has the added benefit of having the polyswitch act as overcurrent protection.

You must NEVER put a MOV from a mains line to ground, only across the mains!!!

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Power strips/surge-suppressors do it (three MOVs - one between each wire).

You cannot have useful answers without discussing the many types of transients. For example, solutions must address both longitudinal and normal mode currents. Furthermore, as medical equipment, you cannot use MOVs or SCRs to dump that energy into the safety ground.

Protection inside equipment involves protection techniques such as galvanic isolation. Medical equipment requires parameters that exceed other requirements. Also means carefully routing or interconnecting different grounds.

Your equipment will be designed to survive small transients. Generally, semiconductor or GDT solutions are better. And not just AC mains transients. A design must address transients from every direction - including from a human touching the front panel.

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

Sez who? what's gonna happen?

ground is the best path to shunt a surge/spike.

I've also seen suppressors using one MOV from Hot to ground and one from Return to ground,omitting the third MOV.

--
Jim Yanik
jyanik
at
localnet
dot com

Eventually, a MOV is going to short (that's how they always fail), put mains voltage on a faulty ground and kill someone. That's why "Y" caps (line to ground) are tested and rated to such high standards (such as

4kV breakdown and extreme reliability under temperature stress accelerated testing).

There are other considerations.

Extremely poor practice, IMHO, but MOVs are dirt cheap in China and I guess it's approved. More reason to avoid cheap power strips.

Yeah, that's what the fuse is for. The

Fuses are cheap, too. I avoid cheap anything, but it's not just cheap Chinese strips that do this.

" snipped-for-privacy@att.bizzzzzzzzzzzz" wrote in news: snipped-for-privacy@4ax.com:

Sez who? what's gonna happen?

ground is the best path to shunt a surge/spike.

I've also seen suppressors using one MOV from Hot to ground and one from Return to ground,omitting the third MOV.

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And what if the ground is disabled? What use is your mov's then?

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The series combo will absorb spikes with amplitudes twice which either
MOV is rated for, but no less.

Not really. If you have a three wire cord, L1 - N - Gnd, some will return from the ground back to the panel neutral. Admittedly, not the ideal.

tm

Safety ground is not earth ground. Many never learn the difference. Shunting transients onto safety ground can make a human patient into a perfect and harmful path to earth. Transients that can overwhelm protection already inside a power supply are hunting destructively for earth. As noted previously, there is no one type of current. And no one ground exists. Properly noted - any MOV in that type equipment can only be for one type of transient - line to line. A transient that approaches (in the same direction) on both wires must not be connected to safety ground.

Medical equipment requires exceeding what is acceptable in other electronics. Any MOV that connects a transient to safety ground may simply be connecting that current harmfully through an attached patient.

Safety ground is not earth ground. Many never learn the difference. Shunting transients onto safety ground can make a human patient into a perfect and harmful path to earth. Transients that can overwhelm protection already inside a power supply are hunting destructively for earth. As noted previously, there is no one type of current. And no one ground exists. Properly noted - any MOV in that type equipment can only be for one type of transient - line to line. A transient that approaches (in the same direction) on both wires must not be connected to safety ground.

Medical equipment requires exceeding what is acceptable in other electronics. Any MOV that connects a transient to safety ground may simply be connecting that current harmfully through an attached patient.

--
you must have missed the part about where the ground was disabled.

''Tis true. Now who would do a silly thing like that? Though I have seen extension cords with the ground pin broken off.

tm

snipped-for-privacy@4ax.com:

You've got two in series, perhaps better than nothing. (mind you I know nothing about MOV's.)

George H.