Much as you can test a fuse by measuring its resistance with a low current and not operate it (**) or indeed a high curretn for a short enough time not to warm up the fusewire, with the right equipment, you can test an SPD by by sweeping the I/V characteristic, but vary fast so there is no significance dissipation, so no internal heating and no migration of material, and so the device does not have its life shortened  by the test.

Indeed you can use changes in the turn on voltage and the rate of rise of current collected by this method, to measure how degraded a device has become by an event, if you have before and after curves for the same device. This allows you to see how well it is performing, and to  exchange the device before it fails catastrophically.

There a very few situations outside a laboratory test facility, where such an approach  is economically justifiable.

Mike.

PS

** for what it is worth, detonators for electrically operated explosives also pass a very low current round the firing loop, to indicate that the bridge wire/fusewire is intact before firing- this is very important in demolition, and other situations where explosive must be fired with close timing and/or in the right order to bring things down safely in the right direction and misfires must be avoided.

For such things a  'no fire' upper safe current, and and 'all-fire' lower limit firing current are a key part of the spec so the safety loop checks and the firing circuit are both reliable..

Ah well, that is because, as yet, I am yet to be convinced that AFDDs actually do anything  especially useful, and so perhaps there no real need to verify if they are still performing as designed....

I would however support their introduction and testing if there was convincing evidence  that suggested not installing and not testing them would cause a significantly increased mortality risk.

For RCDs there certainly is enough evidence to make that case, especially for TT systems..

Mike.

gkenyon said:

Similarly, BS 7671 doesn't require RDC-DDs to be tested - again, manufacturers may have functional test recommendations on installation and for regular maintenance.

Having read a little and watching a couple of videos about the pitfalls of trying to test a RDC-DD in EVSE (edit- with an electricians multifunction or RCD tester) such as those used for charging EVs at home there seems to be two problems.

First, because of the extended period they take to trip the meter may lock out and display over 300 milliseconds indicating a fail when it is actually working properly.

Secondly, again because of the extended period they take to trip, the meter keeps ramping the test current up during the trip sequence, then displays the trip current at the end of the period rather than the current that actually initially started the trip process, again indicating a fail when it is actually working properly.

support.myenergi.com/.../4403885493265-zappi-6mA-DC-protection-to-IEC-62955

Sparkingchip said:

Having read a little and watching a couple of videos about the pitfalls of trying to test a RDC-DD in EVSE such as those used for charging EVs at home there seems to be two problems.

Well, the principal issue is that BS EN IEC 61557-6 does not consider RDC-DD's to BS IEC 62955, it is based around RCDs.

To decide to use an RCD tester for another purpose without checking limitations with the manufacturer of the test instrument (and possibly, in this case, the manufacturer of the RDC-DD under test) is perhaps not a great idea.