Electric shock from phantom power

[Rob Bartholomew]

I thought it night be worth posting an account of this incident to see if anyone else has heard of anything similar.

I work in a music department at an F.E. college. Last week a student came to me saying he had received an electric shock when touching a mic while playing an electric guitar. I checked and there was certainly voltage travelling between the two items of equipment, approximately 50v on my meter -clearly a short to earth somewhere. All the equipment had recently been PAT tested and appeared in good condition. After checking earths on all the equipment to be fine I noticed that a metal music stand with a heavy base had been put down on top of the mic lead and then, obviously, stood on, either very heavily or repeatedly. This had broken the outside of the cable and partially severed the inner cores. It was surprising that the mic still worked but, as long as the cable wasn't moved, at least a partial connection was being made. However, phantom power was turned on at the desk and the return core was shorting to the earth screen in the cable. Thus 48v was going straight to the body of the mic. It only needed a connection to another earth, in this case the metal parts of the guitar, for 48v to be passed through the student. Not lethal and presumably not enough to trip the rcd, but enough to give a bit of a jolt.

I was surprised that such a thing could happen and then, on reflection, surprised that it doesn't happen more. Putting stands on top of leads is clearly not good practice but bit does happen.

Does anyone have any other similar stories or observations?

Regards

Rob

[aidso]

This is not a regular happening but it does happen. Phantom power is 48V at I think max 20-30ma(maybe someone else know the max a better than me). Although not dangerous it is enough to give the jolt. My rule of thumb is "if phantom is not needed turn it off". You can run into problems with some desks using phantom and bad leads.

[Neil Hampson]

I believe that phantom power isgeneraly limited by a 6700ohm resistor on both legs inside the desk, at 48V this gives you just over 7mA if you have been able to short pins 2 and 3 this would be 14mA, just about enough to light an LED. I'm surprised that an artist would complain about recieving a shock at this level. I would check that there are no earth loop issues as well.

[AndyJones]

It might not cause much of a jolt if you were picking it up and handling it etc. But if you are putting your lips up against it, I could imagine that you would notice it alot more?

 

AndyJones

[KevinE]

48V is plenty to give a shock especially if it's handled with sweaty hands. Just ask telephone engineers!!

[p.k.roberts]

Seems a very odd situation; for this to happen, the screen of the mic cable must have been completely severed, isolating the part on the mic side of the break from the part on the desk side of the break otherwise the phantom power would have effectively been shorted as soon as one of the cores came into contact with the screen. Or have I misunderstood this?

[Ynot]

48V is plenty to give a shock especially if it's handled with sweaty hands. Just ask telephone engineers!!

Hmmm...

The 50v line voltage is capable of delivering FAR more that 7mA!

Not a particularly good simile, methinks.

[cedd]

48V is plenty to give a shock especially if it's handled with sweaty hands. Just ask telephone engineers!!

Hmmm...

The 50v line voltage is capable of delivering FAR more that 7mA!

Not a particularly good simile, methinks.

And it can get well above 50v once you ring the line. Believe me, I've been there and it does give you on heck of a crack.

Bridge the 50v battery bussbars in an exchange with something metalic and you won't be walking away. It's not voltage, it's current, or worse a combination of the two.

Two very different scenario's I think. Same voltage, completely different kettle of fish.

[KevinE]

Not a particularly good simile, methinks

 

But it is. The previous poster had essentially questioned whether 50V was capable of being felt. Yes it is. The phantom power in desks is an unpredictable source as different manufacturers generate it in different ways, some use a voltage doubler off an ac winding, some use an LM317 or similar to regulate it off a transformer/rectifier/capacitor. That was the extent to which my 'simile' was used to prove that a shock can be felt off a malfunctioning mic. My contribution to the topic is not to argue about how more or less dangerous the PSTN is compared to a phantom powered mic or how much braver a BT engineer is than a vocalist with a dodgy microphone, it's an illustration of perception of voltage levels, that's all. Please dont jump down my throat for this.

[adam2]

48/50 volts is most unlikely to be dangerous, but could kill in very rare circumtances, such as if a person were trapped and unable to let go, and had a heart defect.

 

The limited current available furthur reduces the risks, but does not totally eliminate risk.

 

Use of an RCD greatly reduces the risks of receiving a mains voltage shock, but would not help in the case of a shock from phantom power.

An RCD measures the current in the live and neutral lines and trips if theses currents are not very nearly equal.

The 48/50 volts refered to above would be derived from a transformer and rectifier within the equipment, and any passage of this voltage through a person would not be detected by the RCD.

 

If phantom power is available then extra care should be taken that leads connected to it are in good condition.

[paulears]

Sticking a 9V battery on your tongue tingles, and placed across your upper lip make you jump, Touching sensitive places with 48V has the same effect. Why are we surprised by this? Leaky spark plug leads on your car hurt too.

 

It's feasible that a damaged cable could cause the problem - the CRITICAL issue is making certain where it's coming from - so you can make make the decision on if it is safe or not. A damaged mic cable is an inconvenience, a damage mains cable is far more serious.

[ojc123]

I'm not looking to over react to this but I have never considered phantom power in my risk assessments. I check the condition of any cable I use and make sure it's ok. I only have phantom on to microphones that need it. I try to keep cables out of harm's way. Can anyone suggest what else I might do to control what seems to be a minor risk?

 

At gig with an external band last year one of the cymbals fell and completely severed a microphone cable. (This was not a bad thing because it was for the drummer's backing vocals. Losing the microphone improved the band significantly imo). I'm a lot more careful where I run cables now. I don't know how far it would have affected a TRS mains flex but I'd rather not find out the hard way.

[Stewart Newlands]

Phantom power can introduce touch voltages in fault or other conditions, this is mentioned in the new BS7909 which has changed quiet a bit from the draft and should now be out before the end of 2008.

 

While not strictly electrostatic this may provide some insight into some touch voltages:

 

BS 5958-1, Code of practice for control of undesirable static electricity – Part 1: General considerations

 

PD CLC/TR 50404, Electrostatics – Code of practice for the avoidance of hazards due to static electricity (available from BSI)

[Simon Lewis]

...... the new BS7909 which has changed quiet a bit from the draft and should now be out before the end of 2008.

 

Any teasers regarding the changes?

[Mixermend]

Wow - a lot of information here - and if the only source of current was the applied Phantom power, certainly no danger existed.

 

In the event of a screen or braid failure (open circuit condition) - firstly you would possibly have a hum problem to cope with. If the braid - on the microphone side of the break - was in contact with one of the cores, then 48 volts would appear on the body of the microphone - and with an absolute short to earth - a maximum of 7mA would flow, or 14mA if both cores were shorted to the braid. There would be no signal from the microphone under this last condition though!

 

So - there would probably be an audible hum as the audio would be pretty unbalanced - and a tingle could certainly be felt if the singers lips touched the microphone whilst holding the earthed guitar strings.

 

So all in all - an unusual fault - but one that is not dangerous.

 

Complaints of tingles when touching microphones are always to be taken seriously though - there are many causes that can indeed be dangerous - don't rely on a recent PAT test (or indeed MOT on your car) - to convince you that all is well. Test and check everything!

 

The telephone situation is slightly different - you can get serious belts from applied ringing, this is around 75 volts at 25 Hz, if I remember correctly - but the normal DC of 48 volts when the 2 wires are shorted together, or indeed broken - will result in much higher back emf's from the line relays used to detect a lifted handset. More than a tingle under these circumstances! The make and break situation was also created when using a rotary dial (remember those?!!) at 10 pulses per second - ouch!

 

 

 

 

A concurrent post has been automatically merged from this point on.

 

On re-reading the original posting - something doesn't quite add up.

 

When you measured about 50 volts - was this AC or DC?

 

If AC (which I am now guessing it was) - then Phantom power is innocent.......

 

Also now you have have time to test the microphone cable - was the braid completely severed?

 

Time to check your mains and safety earths again on everything in use.

 

Please report back.