Rehearsal room/stage electrical safety gear

[Huge Hands]

1 hour ago, TimR said:

 

One band I was in had this scenario. I took the amp home and measured 60volts to earth. It's not static. 

 

It's caused by the 0v of the transformer output not being at Earth potential as it's not grounded via the mains plug.

 

The potential problem is a fault inside the amp causes a fire or the case to become live.

 

An RCD detects an imbalance between the neutral and live. Which you won't get as the 240v side of the transformer is working as expected. 

Hi Tim,

 

I think we are kind of agreeing in different ways.  If the amp was faulty, or causing a PD to its own earth, then yes it would need looking at before it caused itself or the guitarist any damage.  Remember, it is the current that kills, not the voltage, so in some schools of electrical thought, what you read on your meter may have been inside some sort of tolerance (although I doubt it).  Did you get it fixed, or just leave it as was?

 

What I did with the mic cable in my case isolated the guitarist/amp from the PA. so I think, especially as we had a rubber floor on our stage, isolated him enough to stop it hurting his lips or grounding himself in any other way. 

 

I only guessed it was his amp as we didn't have that problem again in the same venue.   However, as I said I vaguely remember him coming the next year with the same amp, so assume he either had it fixed or wasn't faulty either.  

[TimR]

2 minutes ago, Huge Hands said:

2 hours ago, TimR said:

 

One band I was in had this scenario. I took the amp home and measured 60volts to earth. It's not static. 

 

It's caused by the 0v of the transformer output not being at Earth potential as it's not grounded via the mains plug.

 

The potential problem is a fault inside the amp causes a fire or the case to become live.

 

An RCD detects an imbalance between the neutral and live. Which you won't get as the 240v side of the transformer is working as expected. 

Expand  

Hi Tim,

 

I think we are kind of agreeing in different ways.  If the amp was faulty, or causing a PD to its own earth, then yes it would need looking at before it caused itself or the guitarist any damage.  Remember, it is the current that kills, not the voltage, so in some schools of electrical thought, what you read on your meter may have been inside some sort of tolerance (although I doubt it).  Did you get it fixed, or just leave it as was?

 

Yes. The earth wire had been removed from the 13amp plug. I rewired the plug.

 

I also had an incident at a gig where the keys kept cutting out. The keys player realised it was everytime he tapped his foot. Said it would be ok, he'd just keep his foot on the mains plug. I stopped the sound check and insisted on opening it up, and found a very worrying nest of loose wires.

 

Musicians in general are very cavalier with inspecting cables. "If it works, it must be ok."

 

 

 

 

[Huge Hands]

2 minutes ago, TimR said:

Musicians in general are very cavalier with inspecting cables. "If it works, it must be ok."

That is one sentence we can definitely agree on.

 

I do remember working with a pantomime dame who insisted on using his own costumes he had built up over the years.   He had a Christmas tree dress that would be lit up when he walked on stage, then turn off.  I assumed it was some sort of battery thing until half of the lights stopped working and he asked me to take a look at it.  

 

There were fully mains 240V bulbs wrapped around him on 2 circuits.   He would walk out with himself connected to mains, then he would disconnect it by pulling an in-line 2 prong connector apart with one hand and getting the crew to pull the cable back off stage. The two prong plug looked like it was made of bakelite and the socket side could have easily been bridged with a finger when pulling them apart.  When I started going through the dress cabling, I could see where bulb holders had been removed, the wires twisted back together and wrapped (badly) in dried out electrician's tape.   He was a heavy smoker, so a coughing fit and slapping his chest would have probably killed him!   I condemned it there and then and forbid him from using it, which he wasn't very happy about, but I slept better during the run! 

[TimR]

And there we have 4 examples of why PAT Certificates are not just a tick in a box for the insurance. 

 

Venues must see this week in week out. 

[godathunder]

I am a qualified sparky (if somewhat out of practice now). You're both quite right.

 

If there is no earth bond to the chassis/common ground of a piece of equipment then the transformer output is floating, with no reference to the 0v ground/neutral of the supply. Generally, due to an effect called capacitive coupling, it will sit at around half the mains voltage. If you touch this floating "live" chassis you become the reference point to ground. The good news is that, after a small discharge once the reference is made, the capacitive coupling now has a reference to ground and no further current flows. So following an initial unpleasant tingle it is now at 0v. At least, until you remove the reference (ie you let go or remove your lips from the mic etc). Then the capacitance will recharge and another tingle will occur next time you complete this circuit. Roughly translated this means that the effect is a bit like a nearly dead battery that can measure a decent voltage until you connect something to it and then instantly drops to 0v under load

 

While at mains voltages this SHOULDNT prove more than an annoyance, you certainly don't want to be subjected to constant small shocks so the fault, while very unlikely to prove injurious, needs to be repaired. At distribution/transmission voltages (ie National Grid stuff with pylons etc) where you're talking 10s and 100s of thousands of volts, this capacitive coupling carries enough energy to fry you instantly. This vid (at c1min) shows them discharging the line prior to working on it. Note that the line is still live at a very high voltage and the helicopter has no reference to ground but due to the capacitive effect there is still considerably energy discharged. Once it's been discharged, despite the line being live, it is now safe to work on

 

 

There is no harm in cutting the shield cable of a balanced mic/line cable. It's pretty common practice in install work to have all shields only connected to the output side of equipment and disconnected at the inputs. It helps to reduce the number of earth paths and lessens the likelihood of hum caused by earth loops. In fact, you will sometimes find equipment interconnects made with unshielded twisted pair cable such as cat5 or cat 6 with no ill effect (the balancing circuit effectively recombines any interference in reverse polarity which cancels it out)

 

And yes, while I have opinions about PAT, your equipment and mains cables should be inspected/tested, labelled and logged frequently for your own safety and liability - not to mention that some venues will not let you plug in anything not displaying a valid label

 

There's a whole other rant to be made about cheap switch mode power supplies or shoddily manufactured Class II devices but I haven't taken my blood pressure medication yet today so it's probably wise to stay off it for my own safety