Unexpected shock - was it a capacitance issue

[JohnW2]

The switch on the gun should have disconnected the internals from the cable to the plug. Sounds like a poor design that does not meet regulations or common sense.

Yes, I eluded to that in an earlier post. Maybe I'm missing a reason for doing it 'the other way' - but, at first sight, common sense would seem to suggest that any capacitor(s) should be downstream of the tool's switch - not only because of 'safety', but also so that the capacitor(s) were as close as possible to the motor.

Kind Regards, John

 

[DetlefSchmitz]

EMC design is quite a tricky subject. The main cause of interference from most products is down the mains cable, both conducted into the mains and also radiated by the cable. The best location to prevent this is at the point where the cable leaves the piece of equipment, otherwise there is more cable inside to pick up interference.

Putting the capacitors the other side of a switch would not be a safety measure as there is no way to force the user to turn off the switch.

The capacitor values are specified by regulation so as to be unlikely to cause severe shock.

 

[JohnW2]

EMC design is quite a tricky subject. The main cause of interference from most products is down the mains cable, both conducted into the mains and also radiated by the cable. The best location to prevent this is at the point where the cable leaves the piece of equipment, otherwise there is more cable inside to pick up interference.

Perhaps. As I implied, I would have thought that most of the interference would be generated by the actual equipment, not picked up by cable within the equipment. By analogy, 'decoupling capacitors' in RF equipment are always located as close as is possible to the source of whatever is being 'decoupled'.

Putting the capacitors the other side of a switch would not be a safety measure as there is no way to force the user to turn off the switch.

Not foolproof, I agree, but not many people would unplug hand-held tools whilst they were still running, so it would certainly be a large step in a good direction! However....

The capacitor values are specified by regulation so as to be unlikely to cause severe shock.

That's what I thought, particularly for small hand-held tools - which is why I found the whole of this discussion (i.e. the OP's experiences) more than a little surprising.

Kind Regards, John

 

[endecotp]

Peak voltage at 240 V rms = 340 V
Maximum capacitor allowed without bleed resistor = 0.1 uF
Stored energy = 1/2 C V^2 = 5.8 mJ

What does a 5.8 mJ shock feel like?

After much googling:

http://www.chilworth.co.uk/media/121767/a_guide_to_industrial_electrostatics__library_.pdf

the limit of perception is about 1 mJ, and discharge energies less than this will not be felt even in relaxed, domestic or office environments
....
up to a spark energy in the region of 50 mJ. This is a little higher than the worst shock that might ever be encountered simply by getting out of a car and touching it, or by walking across a carpet and touching a door handle or lift button. That is momentarily uncomfortable, unpleasant especially because of the unexpected nature of it, but otherwise harmless in its own right.



So, was the shock in this case consistent with 10% of the worst static shock you might ever encounter?

 

[JohnW2]

Peak voltage at 240 V rms = 340 V ... Maximum capacitor allowed without bleed resistor = 0.1 uF ... Stored energy = 1/2 C V^2 = 5.8 mJ ... What does a 5.8 mJ shock feel like? ... After much googling: .... [in the context of electrostatic discharges] "the limit of perception is about 1 mJ, and discharge energies less than this will not be felt even in relaxed, domestic or office environments".

I think that one problem here is probably that, whilst energy is broadly what determines (above a current threshold) the damage done to a human body by an electric shock, what is perceived as the severity of an electric shock is largely determined by current. Hence, if an electrostatic shock of 1 mJ (driven by a very high voltage, resulting in an appreciable current for a very brief period of time) is 'just perceptible', the same 1 mJ would not be expected to be perceptible if it were driven by a lower voltage, resulting in a lower current for a longer period of time. Similarly, the perceived severity of the 'worst possible static shock' would probably be much greater than the perceived severity of the same energy delivered from a much lower voltage source. Let's face it, if the driving voltage were sufficiently low (and the duration of current flow hence sufficiently long), 50 mJ (corresponding to the 'worst possible static shock') could be totally 'imperceptible'!

I therefore don't really think you can extrapolate from the electrostatic discharge energies associated with the 'limit of perception' and the 'worst possible static shock' to say anything about the perception of a shock driven by an (initial) 340V source.

Kind Regards, John

 

[endecotp]

The similarity with a static shock here is that both are of a short and inherently limited duration. This contrasts with the indefinite duration of a shock resulting from touching a live wire or similar. The tables one can find for the severity of a shock based on current all seem to assume that the shock is applied continuously.

My guess is that the pain for short-duration shocks would be proportional to the energy, or possibly the charge, delivered. But I'm not going to volunteer to test this!

What we really is is for this suspect heat gun to be dismantled!

 

[JohnW2]

The similarity with a static shock here is that both are of a short and inherently limited duration. This contrasts with the indefinite duration of a shock resulting from touching a live wire or similar.

True, but I suspect (and this was the basis of what I said before) that the duration of a static shock is probably very much less than that from the sort of capacitor discharge we are talking about. Of course, I may be wrong (I'm not much of an electrostatics person!).

The tables one can find for the severity of a shock based on current all seem to assume that the shock is applied continuously.

That depends upon what you mean by 'severity of shock'. If you're talking about how dangerous it is to the victim, tabulations and graphs of that will be in terms of both current and duration (hence essentially 'energy', for a given body resistance). The whole idea of our using 30mA RCDs is that a shock current considerably greater than 30mA is fairly unlikely to be fatal if its duration is seriously limited (e.g. to a max of 40ms for currents >150mA, as required of a 30mA RCD).

My guess is that the pain for short-duration shocks would be proportional to the energy, or possibly the charge, delivered. But I'm not going to volunteer to test this!

If one were comparing on the basis of shocks which were not only short duration but also equal duration, then I imagine that would be true, given the relationship between current and energy/charge. However, if you allowed duration to vary a bit, I'm not convinced that a lower current (but longer duration) shock would feel as bad as a higher current (shorter duration) one, even if the energy (or charge) were the same in both cases.

Kind Regards, John

 

[Monkeh]

Numbers aside, I'll just note here that the smashing of the head into the underside of the desk is much more painful than the shock from grabbing a plug connected to such a capacitor. Been there, done that.

 

[mikeyd]

Rocketman: We need pictures. But after you have taken it apart (you know you want to). It's potentially unsafe and needs inspected before you can trust it again

 

[JohnW2]

It's potentially unsafe and needs inspected before you can trust it again

I think that 'unsafe' is probably a bit OTT. Even if it were 340V and a 0.1μF capacitor, and the body resistance about 2kΩ, the effective duration of the shock would only be a millisecond or three.

Kind Regards, John

 

[mikeyd]

It's potentially unsafe and needs inspected before you can trust it again

I think that 'unsafe' is probably a bit OTT. Even if it were 340V and a 0.1μF capacitor, and the body resistance about 2kΩ, the effective duration of the shock would only be a millisecond or three.

Ah but:

Numbers aside, I'll just note here that the smashing of the head into the underside of the desk is much more painful than the shock from grabbing a plug connected to such a capacitor. Been there, done that.

The was intended to suggest that I do not think a disconnected hand held heat gun should be able to do anyone any serious harm.

My badly written suggestion was, that if it was me, I'd take it apart.

 

[JohnW2]

The was intended to suggest that I do not think a disconnected hand held heat gun should be able to do anyone any serious harm. My badly written suggestion was, that if it was me, I'd take it apart.

Oh - in that case I agree with you ... and, yes, I would also take it apart (although I suspect I would learn nothing particularly interesting!).

Kind Regards, John

 

[endecotp]

I actually just tried to take my heat gun apart, but it has a "security" screw that I can't undo! (I have a hex bit that would fit, but it's down a hole that's smaller than the hex.)

 

[JohnW2]

I actually just tried to take my heat gun apart, but it has a "security" screw that I can't undo! (I have a hex bit that would fit, but it's down a hole that's smaller than the hex.)

Ah - would it be a Torx one? If so, you need to borrow my set of Torx screwdrivers - I had to buy them, years ago, because of the same problem you have just experienced - screw down a hole too small to get a hex bit into

Kind Regards, John