Unexpected shock - was it a capacitance issue

[Rocketmen]

Any thoughts as to why I got a belt of my heat gun? I unplugged the gun and was winding up the flex when my hand touched the plug pins. Got a hell of a whack! Plugged it back in again and then unplugged and put a meter across the neutral and live pins - registered 130v and decayed quickly. Is this normal/safe? Not good if I had a dodgy ticker!

Thanks

 

[JohnW2]

Was the blower fan still spinning when you touched the pins (and when you measured the declining 130V)? If not, it really doesn't make much sense.

Kind Regards, John

 

[Rocketmen]

Hello John,
In the first case, the gun had been switched off for around 10 mins before I unplugged I tang got a belt. In the second case, the gun had been switcher off for around 20seconds before I measured what was on the pins.

As you say, it's very odd/not safe.

Thanks for your response,
Andrew

 

[DetlefSchmitz]

Probably just the X capacitor. I get exactly the same if I touch the prongs of my Remington razor after unplugging.

The X capacitor sits across L and N to reduce interference. Naturally it retains a proportion of the mains voltage for while. I have got into the habit of just not touching it.

 

[JohnW2]

Hello John, In the first case, the gun had been switched off for around 10 mins before I unplugged I tang got a belt. In the second case, the gun had been switcher off for around 20seconds before I measured what was on the pins. .... As you say, it's very odd/not safe

If there is a persistent measurable (or feelable) voltage when the motor is not rotating, I would say that 'very odd' is a serious understatement, but I doubt that it is 'unsafe'! Maybe someone else will have some clever ideas!

Kind Regards, John

 

[JohnW2]

Probably just the X capacitor. I get exactly the same if I touch the prongs of my Remington razor after unplugging. ... The X capacitor sits across L and N to reduce interference. Naturally it retains a proportion of the mains voltage for while. I have got into the habit of just not touching it.

10 minutes after unplugging? Even if one were lucky/unlucky in disconnecting power close to the peak voltage point of the cycle, it would take a very low leakage (aka 'unnecessarily expensive'!) capacitor to retain a significant voltage for 10 minutes, wouldn't it? ... and that's assuming that the capacitor is upstream of the switch (rather than directly across the motor).

Kind Regards, John

 

[DetlefSchmitz]

Well... he said 10 minutes after switching off, but just after unplugging.

 

[JohnW2]

Well... he said 10 minutes after switching off, but just after unplugging.

Ah, so he did - I should learn to read more carefully In that case, you could well be right (for the first few seconds).

Kind Regards, John

 

[davelx]

I get this from my blender - it's the suppressor capacitors, generally 3 connected in a delta configuration between L, N & E in a single housing, which can hold a charge for quite a while if they're not the type with bleed resistors across them.

I can get a pretty good spark by shorting the pins of the plug, for a fair while after unplugging.

 

[Rocketmen]

Thanks guys, the gun seemed to have held the charge after switching off the unit and unplugging it at some later time. Was confused as to how I could get a belt off something that was disconnected but also worried from a safety point of view.

regards
Andrew

 

[endecotp]

BS EN 60335-1 2002

22.5 Appliances intended to be connected to the supply mains by means of a plug shall be constructed so that in normal use there is no risk of electric shock from charged capacitors when the pins of the plug are touched.
NOTE Capacitors having a rated capacitance up to 0,1 μF are not considered to entail a risk of electric shock. Compliance is checked by the following test.
The appliance is supplied at rated voltage. Any switch is then placed in the off position and the appliance is disconnected from the supply mains at the instant of voltage peak. One second after disconnection, the voltage between the pins of the plug is measured with an instrument that does not appreciably affect the value to be measured.
The voltage shall not exceed 34 V.

 

[Rocketmen]

Thanks Endocopt, it looks as though my gun fails this test.

Regards
Andrew

 

[mikeyd]

It might be that the design includes a discharge resistor which has gone open circuit, or has a dry joint to the circuit board. http://en.wikipedia.org/wiki/Bleeder_resistor

 

[bernardgreen]

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.

 

[ricicle]

A lot of household appliances do this, especially blenders, hairdryers etc. Generally those with motors in them.