Over voltage Protection

[Scutarius]

Hi, I am involved in the design of an equipment.

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

 

[vk6zgo]

Cheat!!
Have a look at some other manufacturer's equipment & see how they do it.
If their equipment meets the required spec,you should be able to do something similar to do the same job.

Over many years,I have found that different companies tend to use very similar designs in many parts of their equipment,as much of this stuff is in the public domain.

VK6ZGO

[Psi]

i don't really want to try an give answers on medical equipment and safety because i don't really know anything about the subject.
So don't take anything i say as fact, but i have a few thoughts on what they may mean.

When they say "the OVP must not disconnect the device from the mains"  they may mean that the device must not switch itself off as that would be unsafe if it was some sort of life support system etc.. The device would be quite unsafe if it "disconnected itself from the mains" as it would be unable to sound any alarm to let people know it wasn't running any more.

There are a few ways to design a system that is able to keep running in the event of a failure, having dual or triple circuits is one way. For example, dual power supplies where one can fail, and sound an alarm, without affecting the operation of the device.

As far OVP goes, if your trying to absorb spikes from the mains, or from the supply, a TVS diode can be used to just absorb the spike without disconnecting anything. There are TVS diodes that can absorb kilowatts of energy for very short periods. 
But If your OVP is to protect the rest of the circuit from an out of control power supply that might supply continuous over voltage then a TVS diode isnt going to work. I guess in that situation you need dual supplies.

But again, i'm no expert on medical equipment, you really need to talk to someone who knows about the subject. 

[A-sic Enginerd]

This is where I make the statement: "my ignorance will allow me to say something that is quite possibly really moronic."

Now with that out of the way......

How about basically a built in UPS if you will. Maybe a combination of SCR and fuses as you said. See a spike, maybe you blow a fuse. After the event and stable mains are there again, maybe do like Psi suggested and have a backup power supply you then switch in. In the mean time, the system runs off a small built in UPS (ie: large cap).

I'm still getting my feet wet with this level of analog, so my knowledge is waning. However, the digital chips I've been working on the last few years have been all about maintaining data integrity on the storage network in the enterprise space. Details of the problem are different, but when you distill it down to it's most basic concept it becomes: keep the system alive no matter what. To solve that, one of the most basic rules has always boiled down to redundancy redundancy redundancy.

[saturation]

The principle is simple: life critical electronic equipment are designed to be battery powered and the mains is there to charge the batteries.

Its like UPS, but unlike a UPS, the device is actually designed to operate for many hours without the mains.

When an OVP event occurs and if the protectors have tripped, an annunicator should mark that the protective circuitry have been triggered.  If they are one-shot and cannot be reset, an alarm should sound tied to the devices main batteries.  If they are self resetting, then optionally send a signal to the main unit that can count the number of OVP issues; in self-resetting units the device is still connected to the mains, but if the fault persists, eventually the OVP circuits will fail so the device must be disconnected from mains but still function, and thus the battery power.

MOV devices are single use when triggered, but they can be designed as a replaceable module that can be swapped out when triggered.  The whole charging unit can also be designed as a module.  In health care settings, OVP is built into the main distribution and every branch circuit, and even on single outlets.  The devices OVP is a last resort.

[tecman]

You can survive quite a bit with good design.  This means normal and common mode inductors on the incoming power.  This is followed by MOVs, solid state devices (Transzorbs) and gas discharge protection.  The suppressor can be one or more of these devices.  You will need a model of the incoming spike to properly select the size and rating of the suppression device.  Remember that the telco POTS system takes all type of surges and keeps on working.  Inductors are made for these applications and are your friends (remember dV/dT).

paul

[Zero999]

It depends on the level of over-voltage (maximum peak voltage, time and energy) and the nominal mains voltage. It's fairly easy to protect against short transients with MOVs, diodes and gas discharge tubes but protecting against a sustained over-voltage such as that created by a faulty regulator in the power company's transformer is more difficult.

[Scutarius]

Thank you guys,  great answers.

We are going to start working in your advises.

[Bored@Work]

You need to get the relevant standards and regulations for the juristication where the device should run. Because these form an important part of your requirements. It is not enough that your device can handle some overvoltage. In needs to be able to handle at least what the standards and regulations require.

A second reason is that some juristications require that you create and keep certain documentation while you develop about how you develop following certain rules. You can not create them after the fact.