Will a surge protector work both ways?

Posted by: @apeshaft

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First of all I would like to apologize if some of the words and terms are used incorrectly or misspelled since English is not my first language and many of the terms and words used are very different in English than they are in Swedish.

My question is about surge protectors that you connect to your wallsocket and then plug-in your computer, for instance, into the surge protector to avoid damage during lightning strikes nearby or power surge in general. Do they only function in one direction? So, if a power surge comes into my house, through my fusebox and out through the outlet where the surge protector goes off and prevents my computer from getting fried.
But what if I instead connect to something like a battery charger or an electric fan that may have been shorted out at some point? If I plug it in into the surge protector, will the surge protector go off

They work both ways.

To test if a device is shorted out it to place the VOM in resistance mode or if it has a continuity checker use that. Depending on the device it should have more than 0 resistance. At a minimum, divide the supply voltage by the rated current, but that is only true for resistive loads while motors are quite different. In any case, a short circuit is very different from a power surge, if you have working breakers, then plugging a shorted-out device into a socket is an acceptable test.

Hi @apeshaft,

   The difficulty of answering questions like this which refer to a 'type' of product, is that there are often different products with similar descriptions. This answer applies to what I believe is the 'common' approach.

A surge protector will try to absorb an over voltage pulse, regardless of whether is coming from the plug or the socket of a typical distribution board. Of course, it is unusual for a device that consumes power, like a fan, to suddenly become a surging power source.

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I think you may find it helpful to understand how a 'surge protector' works ... I'll try to explain ...

You may be familar with a device called a Zener diode ... https://en.wikipedia.org/wiki/Zener_diode

If you have a fixed voltage dc supply, then (in appropriate circumstances) it is possible to use Zener diode as a kind of surge protector ... you basically pick a Zener diode whose breakdown voltage is a little higher than the 'normal' DC voltage ... say 15V Zener for a 12V supply. Then if the voltage 'surges' above 15V, the Zener diode will start conducting, and attempt to clamp the voltage to just above 15V.

Of course, when the Zener is conducting, it may potentially carry a high current, become very hot and expire if the power in the surge is greater than it can physically cope with.

Hence, some manufacturers produce devices with a similar characteristic, but specially designed to cope with a larger 'one-off' transient. These are often known as TVS - transient voltage suppresors.

This reference provides a simple overview:

https://www.allaboutcircuits.com/technical-articles/transient-voltage-suppressors-tvs-an-introduction/

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I suspect you are actually referring to mains power 'surge protectors' ... which obviously must deal with higher voltages and alternating current, so we need to expand the discussion.

In principle, you can use two Zener diodes connected in series, with the two anodes connected together and the cathodes connected to the mains power lines. Hence, for each half-cycle, one of the diodes will be conducting, but the other diode will not, so the current flow will be zero, unless the voltage exceeds the Zener voltage of the non conducting diode during a power surge. If that happens, the two diodes will start to conduct in an attempt to clamp the voltage.

Again, two Zeners would be limited in their physical capability of conducting the power in a large surge, so in practice, specialised devices with similar characteristics are available.

One company that I remember that specialises in both DC and AC devices is Littelfuse, if you wish to research further. (Other companies also produce them.) I normally try to avoid recommending products, but Littelfuse do produce some of the best free leaflets on the subject, although they can be a little tricky to track down! e.g.

https://m.littelfuse.com/~/media/electronics/trainings/littelfuse_varistor_mov_products_training.pdf.pdf

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Note that the terms TVS and MOV are usually associated with different underlying technologies, for devices with roughly similar characteristics. From memory, the MOV devices are easier to manufacture and hence tend to be cheaper, but have a limited life if regularily exposed to large transients, as each transient 'ages' them. This may not apply to all device types, but is something to check for when designing with them.

(NB - I think Littelfuse also have a different leaflet for TVS devices.)

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I note you are discussing shorted components, though I am not clear about the context. Note TVS and MOV devices are purely capable of clamping a high voltage spike ... such as when a power line is hit with lightning, .. whereupon they will attempt to prevent the voltage rising high enough to damage any connected loads, etc., by absorbing the excess power. They have no influence on the current drawn by a device, nor are they concerned with a load that has developed a short circuit - that is a job for fuses and circuit breakers.

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Hope this helps.

Dave