Electric Shock

Dubh Geannain

Apologies if this is the wrong forum but I'm looking to pick the brains of some tech heads.

Was sitting on my bed last night strumming away on my electric guitar, plugged into an amp and headphones on. I had my ipod across from me syncing with my laptop at the time and I went to move it a little. Upon touching the (metallic) back of the ipod: BANG I get a bad electric shock. Just to give an accurate account I touched the ipod a split second before this also and the mains hum in my earphones increased.

Funnily enough I work in electrical safety but this one has me stumped and I know the difference between a static shock and a proper electrical shock. The USB circuit connected to the IPOD is an isolated Safety Extra Low Voltage circuit meaning it's seperated from earth by at least basic insulation. Basic insulation generally has a breakdown voltage of greater than 1000V, so it couldn't have been the result of being earthed via the ipod that caused the shock. So what did? The guitar itself is just over a month old by the way and I wouldn't be expecting any faulty connections with it. I'm guessing the voltage in the first place would have been generated by one of the pickups but I'm really only a novice whe it comes to electric guitars, so I don't know what kind of voltage sits on these. Any ideas there folks?

Cheers

Mods move to the appropriate forum if this is the wrong place

AlmightyCushion

Moved from Comp and Tech. More suited here so you should get an answer.

Andrea B.

Funnily enough I work in electrical safety but this one has me stumped and I know the difference between a static shock and a proper electrical shock.

What was the aftermath?

Was it continuous, or did it dissipate after you touched it?

Don't underestimate static by the way.

steifanc

i get shocks from the guitar , its normally a loose wire in the lead, you probly earthed your self off the i pod, the i pod wouldnt have egnough power to really feel a shock from

Stoner

was the laptop plugged in, just tio get the chain sorted, is this correct it might not be but its the case with the most electronics involved.

1) Power socket on the wall into the amp
2) Amp connected to the guitar
3) you sitting on the bed feet up using the guitar
4) Ipod on the bed, linked via usb into the laptop
5) Laptop plugged into the wall power socket charging.
6) you holding the guitar with one hand; reach over and touch the case of the ipod---- shock

Were any of these situation not in the circuit?

davelerave

sounds like static.separation of charges . hopefully someone can explain.i don't have any in-depth knowledge unfortunately

Mike 1972

Laptop plugged into the wall power socket charging.

A lot of cheap switch mode power suppy units (and TV sets) have poor isolation. Even if the DC output is only a few volts there is often an AC component of anything up to half mains voltage (albeit current limited so that it isint regarded as dangerous ebven though its a nuisance) With computers or any type of AV equipment (i.e. with signal inputs/outputs) this can lead to unexpected voltages appearing in all kinds of palces. Another hazard is stored charge appearing across the live and neutral pins of a plug even after it has been unplugged from the mains. The fact that such junk is widely sold points to inadequete (or unenforced) product safety regulations.

Dubh Geannain

Stoner wrote: »

was the laptop plugged in, just tio get the chain sorted, is this correct it might not be but its the case with the most electronics involved.

1) Power socket on the wall into the amp Amp plugged into a 4 gang adaptor
2) Amp connected to the guitar
3) you sitting on the bed feet up using the guitar Feet on ground, waering runners
4) Ipod on the bed, linked via usb into the laptop
5) Laptop plugged into the wall power socket charging.
6) you holding the guitar with one hand; reach over and touch the case of the ipod---- shock

Were any of these situation not in the circuit?

That was pretty much the situation.

Andrea B. wrote:

Don't underestimate static by the way.

I'm almost sure it wasn't static. The shock lasted too long. Had it been static as in a couple of thousand volts I think I would have recognised it.

Mike 1972 wrote:

A lot of cheap switch mode power suppy units (and TV sets) have poor isolation.

This could be the case although it hadn't happened to me before until I was using the guitar. The power supply for my laptop is actually a replacement for the original power supply. I got it off e-bay.

So here's my thinking at this stage. The SELV circuit (that is the IPOD) drifted up to at least half mains. Ordinarily when I would touch it my resistance is probably too great to allow a flow of charge to earth. It's possible that I earthed myself via the guitar strings (if they are earthed via the amp; don't know enough about electric guitars tbh). So the charge from the ipod circuit flowed to earth through me and not from the guitar/amp to the ipod circuit.

Stoner

Dubh Geannain wrote: »

So here's my thinking at this stage. The SELV circuit (that is the IPOD) drifted up to at least half mains. Ordinarily when I would touch it my resistance is probably too great to allow a flow of charge to earth. It's possible that I earthed myself via the guitar strings (if they are earthed via the amp; don't know enough about electric guitars tbh). So the charge from the ipod circuit flowed to earth through me and not from the guitar/amp to the ipod circuit.

Sounds very reasonable alright, it's funny how we can put ourselves in a circuit. I remember testing PSUs in Philips wrt to all kinds of issues, including knocking it of a table, baking it, electro magnetic suppression and zapping it with a static discharge gun. I never realised until i went there just how much work goes into making sure items are safe to a standard (and how much testing/checking and rechecking), so a PSU of ebay could very well be an issue alright.

Dubh Geannain

Stoner wrote: »

Sounds very reasonable alright, it's funny how we can put ourselves in a circuit. I remember testing PSUs in Philips wrt to all kinds of issues, including knocking it of a table, baking it, electro magnetic suppression and zapping it with a static discharge gun. I never realised until i went there just how much work goes into making sure items are safe to a standard (and how much testing/checking and rechecking), so a PSU of ebay could very well be an issue alright.

https://www.youtube.com/watch?v=JxEVnZKnxyg

Not exactly safety related but thorough all the same.

Stoner

i wonder if he sits on the phones?