Shock from amp...after being unplugged

digitalscream

Have you ever had an electric shock from an amp *after* you've unplugged it from the mains?

It's happened to me a couple of times now, with my Seymour Duncan Powerstage 170. Unplug from the mains, but accidentally touch the pins on the mains plug a few seconds later and BLAM! Shock all the way up my arm.

It's getting a bit disconcerting, if I'm honest. Don't want to have to return it, because it's a brilliant amp (aside from trying to kill me every now and then).

ICBM

Yes - surprisingly, it's normal with amps with a mains spike filter in the IEC input socket. This has caps in it which retain charge, at up to the full mains voltage depending on which exact place in the AC waveform the power was switched off at! The current they will deliver is tiny though, so although it's painful I don't think it's dangerous. It is a rather remarkable oversight in the regulations though.

If the amp has an IEC cable, always unplug it at the amp end first. The pins inside the socket can then still give you a shock, but you're unlikely to touch them.

ennspek

As above. Some hold charge. Glad you're ok.

MichaelAT

Hm, why would you touch the pins on the mains plug? Seems I was lucky, never happened to me. +1 it's by design (voltage stored), that's why there are always like 50 disclaimers you should never open up the amp and work inside if you don't know how to discharge the caps. 
When you turn the amp off and keep playing, you still hear the amplified sound, slowly fading. With no power stored, the amp would be immediately silent.

ecc83

The mains RF filter cap is fitted to most pieces of kit but the designers don't think peeps are going to grab the plug pins!

Sometimes the cap is post mains switch and so if you switch off there is no connection. The problem with that is if the cap shorts* it can weld the switch up!

As mentioned, hurts but not dangerous..Well, be careful if you have a pacemaker!

*These are special "X" or "Y" rated capacitors and can "self heal" i.e. they can short but recover, the shorted section is blown away.

Dave.

sgosden

Capacitors are sneaky. 

clarkefan

I don't understand why it's not dangerous.  Why isn't this as dangerous as working inside an amp with caps that are still charged?  Same voltage and current surely?

Danny1969

clarkefan said:

I don't understand why it's not dangerous.  Why isn't this as dangerous as working inside an amp with caps that are still charged?  Same voltage and current surely?

No, energy is measured in joules ..... we come across high voltages all the time, static voltages can be way higher than you would find in an amp BUT their duration when shocking you is a tiny fraction due to the tiny amount of stored energy in Joules 

A small cap is the same deal .... a bigger cap like a reservoir capacitor used to smooth the DC supply IS capable of holding enough Joules of energy to kill you ... 

clarkefan

Thanks for this, and I appreciate I'm being thick, but I don't understand.

If I'm working inside an amp that hasn't been discharged and I touch something I shouldn't the caps will discharge through me and I could die.

Why do I not die touching the plug pins?  Am I not giving the charge in the caps exactly the same path to ground?

ICBM

clarkefan said:

Thanks for this, and I appreciate I'm being thick, but I don't understand.

If I'm working inside an amp that hasn't been discharged and I touch something I shouldn't the caps will discharge through me and I could die.

Why do I not die touching the plug pins?  Am I not giving the charge in the caps exactly the same path to ground?

The amount of charge stored is far smaller because the cap value is far smaller. The current produced is the rate at which that charge flows through you, and hence is also far smaller - and it's the current which is dangerous. The amount of charge will decay to zero before a lethal current can be delivered.

Even so, the shock from a spike suppressor is a lot more painful than a static shock because there is still more charge stored. But the main filter caps in an amp are a huge value by comparison - at least 100 times larger - and store easily enough charge to produce a lethal current for long enough to kill you.

digitalscream

ecc83 said:

The mains RF filter cap is fitted to most pieces of kit but the designers don't think peeps are going to grab the plug pins!

An understandable assumption (ish), but it kind of defeats the point of the safety aspect of male/female plugs

Also, it's fine if the charge dissipates quickly, but for it to still be there 10 seconds later isn't good design IMO. Sure, I can unplug from the device first, but the bassist in our band picked the amp up (remember, it's only a small box) and accidentally got fingers near the mains socket and got the same shock when it shifted in the hand.

The only real preventative, I suppose, is to stop using the power switch on the amp and use the mains socket switch instead so the caps discharge instantly.

Danny1969

clarkefan said:

Thanks for this, and I appreciate I'm being thick, but I don't understand.

If I'm working inside an amp that hasn't been discharged and I touch something I shouldn't the caps will discharge through me and I could die.

Why do I not die touching the plug pins?  Am I not giving the charge in the caps exactly the same path to ground?

Well lets imagine that to harm you it takes a high voltage that's capable of supplying enough current for a certain length of time. That's 3 factors 

Voltage potential
Impedance of voltage potential
Amount of storage 

To cause a serious shock it takes around 20 to 30mA of current for a certain length of time .... than means there needs to be a high enough voltage to push 30mA through the impedance of  your skin and flesh, the voltage source needs to be a low enough impedance to source 30mA to achieve this  and finally it needs to be able to supply 30mA for long enough for you to be harmed .... lets say 1 second instead of 10ms

So a small cap although charged to a high voltage isn't holding enough energy  to do this ... in the same was the static charge stored on your jumper isn't either

Danny1969

Until very recently your average laptop contained much higher voltages than a valve amp ..... the inverter used to power the backlight gave out anything from 600 to 900V  .... as laptop engineer for many years I had many a shock but no harm as the inverter couldn't supply enough energy if that makes sense. Bit like the HT circuit on a car, survived many a shock of a coil or distributor  over the years working on cars .... that's thousands of volts 

ICBM

digitalscream said:

The only real preventative, I suppose, is to stop using the power switch on the amp and use the mains socket switch instead so the caps discharge instantly.

Edit - ah, I read that wrong! Yes, switching off at the wall first not using the amp's power switch should prevent it.

skunkwerx

Is it not still dangerous though for people with heart conditions or, not knowing they have a condition?
Would a healthy adult be able to stand mild levels that would other interrupt another heart? 

Genuine question that entered my mind! 

ICBM

skunkwerx said:

Is it not still dangerous though for people with heart conditions or, not knowing they have a condition?
Would a healthy adult be able to stand mild levels that would other interrupt another heart? 

Genuine question that entered my mind! 

Possibly. I have to admit I'm a bit surprised it's allowed - the shock you get isn't a minor 'static' like spark, it can be quite a painful belt.

digitalscream

skunkwerx said:

Is it not still dangerous though for people with heart conditions or, not knowing they have a condition?
Would a healthy adult be able to stand mild levels that would other interrupt another heart? 

Genuine question that entered my mind! 

Given the proximity of the pins to each other, I'd suggest it's not that dangerous - because the current would be highly unlikely to travel across the chest (you'd be touching them with the same hand). If you were to touch live and neutral with different hands, though...I guess it's a possibility?

I've had a few shocks in my time, and this one definitely sits at the "hell of a jolt" end of things.

RiftAmps

ICBM said:

 It is a rather remarkable oversight in the regulations though.

@jpfamps do you know if the new regs are addressing this?

skunkwerx

Ah, yeah does seem strange!

ecc83

It is perhaps worth mentioning that it actually takes a lot of electricity and for a long time to kill a person as they found out in a most horrible way with the first use of the electric chair!

What a short, sharp shock can do if it goes across your chest is put your heart into "fibrillation" a sort of non-effective fluttering I understand. You then die from the heart attack if help is not immediately to hand.

There also seems to be some confusion about the current needed to cause fib'? I was always told "Five mils kills".

There was a Mythbusters prog about the "heater in the bath" Bond incident and they started off looking for 40mA or so? Later they were advised that 5mA was the lethal current.

Re the cap across the mains. Sometimes a 1meg bleed resistor is fitted but,

1) That value still takes a second or two to discharge the cap.

2)Very high value resistors under constant high voltage stress have a habit of going O/C!

Dave.

Fenderish

in good old amps, the caps are after the mains switch, so if you put the mains switch on off, you can never be schocked by unplugging the cable. on modern amps, such things as a mains supply board with the caps is still on the mains after having switched the amp off, Don't like that...