Shocks from mic

[ian2]

On Sunday my wife reported feeling a tingling electric shock when her lips touched the mic into which she was singing at church.

The setup was somewhat different from usual. Rather than me playing electric guitar and being led by our usual worship leader, with his vocals and all-singing keys (which can provide drum sounds and bass sounds) he was absent and I was leading on electro-acoustic guitar and vocals.

We (sadly) don't usually have any foldback whatsoever. However, on Sunday I took my wedge monitor, which was fed with a mix of my electro-acoustic guitar, my mic and my wife's mic:

Code: Select all

Mic1 ==> DI_1 ==> Snake ==> FOH
	||
	||==> Ch1 of on-stage mixer
	||			||
	||			||==>Sole wedge monitor
	||			||
	||==> Ch2 of on-stage mixer
	||
Mic2 ==> DI_2 ==> Snake ==> FOH

Electro-acoustic ==> DI_3 ==> Snake ==> FOH
			||
			||
			||========> Ch3 of on-stage mixer ==> Sole wedge monitor

So, the non-usual equipment that we were using were essentially the on-stage mixer - my Phonic MM1002 - and my wedge monitor, a Laney CP12. I should also mention that, in order to split the mic signals on-stage, to feed both FOH and foldback, there was the unusual configuration of having DI boxes between the mics and the snake.

For info, the FOH mixer is a Peavey 18 channel thing, and would have had phantom power enabled, in order to power the DI boxes, which are Behringer DI-100. The mics are Sennheiser dynamics.

Although I've consequently purchased an RCD, and resolved always to put it between my 4-gang power adapter and the wall socket, there is surely still an electrical fault somewhere. I haven't yet assimilated all of the mic shocks sticky post found within this section of the forum, but our church minister has been informed of the issue.

This evening I tested for continuity between the chassis of my mixer and the earth pin of its main plug. No continuity. The manual states "This product is equipped with a 3-wire grounding type plug; this is a necessary practice to prevent electric shock hazards to the operator, the microphone user, and the musicians who are wired to this unit." Nonetheless, taking the mains plug apart I found only two wires; one connected to live, one to neutral and nothing connected to earth. Further inspection yielded the double insulated symbol on the Phonic power supply that came with the mixer. Presumably that part of the manual is incorrect, perhaps having been copied from the manual of an older model.

I'd appreciate your thoughts on the above, and your wisdom in identifying next steps in the investigation. I can think of several, but don't have the expertise to know how to prioritise them (and I'm sure there will be additional steps to take):

• Testing for a good connection from the wedge monitor to earth. However, I don't know what exposed part of the wedge, if any, ought to be at earth potential.
• Testing the earth connections of the other components.
• Another church member has a device which can be used to test the room's mains sockets.
• Testing my 4-gang power adapter's earth.

[Hugh Robjohns]

There are two types of 'shock' in situations like this.

The first, and benign form, is a short, non-continuing zap caused by discharging static electricity. This is most usually generated by clothes/shoes and discharged by touching a grounded mic. You get repeat shocks if you stop touching the mic, move around to create more static, and then touch the mic again.

The second — and much more worrying form — is a continuing 'tingle'. This is much worse in Europe than in the USA because it's normally caused by the system's metalwork rising to half mains voltage due to a missing safety earth connection. So the mic body becomes 'half-live' and effectively discharges to ground continuously through the user when they touch it.

Normally, this is unpleasant rather than lethal as the impedances are quite high... but the missing safety ground means that a second fault or failure could make the whole system a real killer! So it is vital to find and fix the problem.

An RCD is always a good idea — I never plug anything into a wall socket at a gig without one — but it may not trip in situations like this.

In your case, I suspect the DI boxes have isolated the mics and your gear completely from the FOH system (which I presume is properly grounded).

But the ungrounded Phonic mixer is connected directly to the mics, allowing their bodies to rise to half mains voltage.

As the hub of most audio systems, mixers are usually grounded. But it seems that's not the case here. Often a double insulated mixer like yours will get grounded through other connected equipment, such as a powered wedge monitor, so you might want to check the grounding of all your equipment.

It must also be said that a real oddity here is the use of DI boxes as mic splitters! Thats definitely NOT what they're intended for. They will typically knock 20dB off your mic level to FOH and add significant noise. You'll also have an unbalanced feed to your local mixer which might result in unwanted interference.

You'd be much better off using proper mic splitter boxes (with the FOH using the Direct connection).

Alternatively, connect the mics direct to the FOH console, and setup prefade aux sends from the mic channels to provide line level monitor feeds back to your local mixer.

But definitely check for missing mains safety grounds on all mains cables, plugboards, IEC leads (etc) and equipment. And then check your audio cables have intact screens too.

[ef37a]

Hi Ian, you must of course get the whole system checked out for safety possibly to the extent of getting in an electrician to check earth impedance of the mains supply. ANY case of any kind of shock, however mild must be investigated.

However, I think the sources of the "tingles" your wife felt can be explained by the fact that the Phonic mixer supply is class 2, double insulated. This sort of gear can 'float' at about 1/2 mains potential and a very mild shock or tingle can often be felt. Common on the metal parts of certain laptops for example.
The source resistance of this voltage is very high, capacitive in fact and so could probably be eliminated by tying the screens of the mic XLRs to a good earth with a few k Ohms.

You could just bond the chassis of the mixer solidly to earth but that is likely to throw up ground loop hum problems.

More and more audio equipment is going 'earth free', monitors e.g. and I can see more posts coming about people getting mild shocks and hum problems.

I am sure Hugh and other better men than I will comment very shortly.

Begger me! You're quick HR!

Dave.

[shufflebeat]

It's my understanding (happy to be corrected) that an RCD is only effective if everything is going through it, i.e., on the first distribution point of a star system or on every socket if several are being used. Any socket (outlet) without one negates the whole process.

No?

[Hugh Robjohns]

Yes.

An RCD checks that the current on the Line (Live) exactly matches that on the Neutral from the wall socket.

If it doesn't, it can only be because current is going to ground 'illegally' — possibly via an unfortunate person — due to a fault.

So, if that imbalance condition is detected the RCD trips its mains output power very quickly and hopefully before the 'unfortunate person' gets fried.

Ideally, everything should be powered through a single RCD trip, but in more complex rigs each device (or group of devices) can have its/their own RCD(s).

But if some devices are powered via RCDs and some aren't, the whole system safety is severely compromised since in the event of a major fault some gear will remain powered even if the RCDs trip out, potentially resulting in personal frying...

[Mike Stranks]

I have met Ian a few times in connection with my previous church-sound work, although haven't seen him for several years now. As his post indicates, I know him as a meticulous and careful person who knows his stuff and wouldn't be cavalier about anything.

But - and not a pop at Ian - his post indicates that there is still a great deal of uncertainty and awareness of the differences between, and uses of:

* DI Boxes
* Iso-transformers
* Splitters - esp mic splitters

I think we've had several Hugh-written tutorials in the Forum about the differences and uses and, I think, something at some point in the Technical Q&A in the magazine.

Time for more of a feature in the mag about the important little boxes you might need on the road and the differences and uses?

(I now await one of m'learned friends quoting chapter and verse of an article that's already in the archives! )

[I always used mic splitters when doing location recordings for local radio - the venue had the transformer-side for FOH if they wanted it. FOH and recording systems completely isolated from each other following a couple of pre-recording nasties (quickly sorted) in my early days of doing this.]

[Drew Stephenson]

Add re-ampers to that list as well I think Mike.
I've seen some 'interesting' attempts to press them into unconventional service.

[Hugh Robjohns]

I've seen some 'interesting' attempts to press them into unconventional service.

Ooh! Pray tell...

H

[Drew Stephenson]

Using them back to front with a hodge-podge of adaptors to try and make up for a lack of suitable inputs on a mixer being the one that springs to mind.

[ian2]

Thanks for everyone's inputs so far.

We were back to a more typical set-up for today's service. However, before we started:

• I used a newly purchased T.I.S. 1005 socket tester (no-one in the UK seems to have stock of the Martindale's near equivalent EZ165, at present) to confirm that three green lights were indicated for each of the room's mains sockets that we would be using to power the PA and everything connected to it.
• I inserted a Masterplug RCD into each of the mains sockets that we would be using to power the PA and everything connected to it.

I note the warning that RCD's may not trip in the scenario that we had last week, and understand that there are a lot more more checks that can be done, including:

• (Quoting Hugh's post): "definitely check for missing mains safety grounds on all mains cables, plugboards, IEC leads (etc) and equipment. And then check your audio cables have intact screens too."
• Use the T.I.S. socket tester to test that each RCD trips when it's supposed to.

I heard from our usual worship leader today that he has previously experienced shocks when he has touched his mic. He believes that the shocks that he experienced were static shocks. However, my wife believes that, rather than a "zap", what she experienced last week was a tingling which continued until she moved her lips away from the mic.

Some questions:

1. Hugh, is it the case that your warning that RCDs may not trip in last week's scenario stems from the likely physical decoupling of electrical circuits brought about by the way that DI boxes (which I understand to contain transformers) were being used in the system?
2. Hugh, you wrote: "the ungrounded Phonic mixer is connected directly to the mics, allowing their bodies to rise to half mains voltage". From a physical perspective, the Phonic mixer was connected indirectly to the mics, via the "LINK" socket of the Behringer DI100 DI boxes. But have you written "directly" because you either suspect or know it to be the case that the DI boxes' "LINK" outputs are directly physically connected to the XLR input socket? How can the bodies of the mics rise to half of mains voltage?
3. Suppose it were known for certain that the shocks that my wife experienced were "only of the static variety"; would it nonetheless be the case that:
   1. The shocks were an indication of the lack of a good connection to earth somewhere in the system
   2. The lack of a good connection to earth somewhere in the system would have amounted to a system in which one further fault could make the system potentially lethal?
4. https://service.shure.com/Service/s/art ... uage=en_US tells us that a possible reason for an electric shock from an SM58 (which is not the vocal mic that we use at church) is lack of a good connection between the grill and pin 1 of the XLR. Suppose the grill of such a mic has become partially unscrewed from the mic's shell, such that the threads mesh loosely and the connection between the grill and earth is poor; how many faults away from a potentially lethal state could that, by itself, be? One? Or none?

[James Perrett]

In answer to a couple of questions...

The link sockets are connected directly to the input sockets on the DI. The input socket is not an XLR - that's the output socket which may (or may not in some cases) be isolated from the input.

If it were static, you would feel a quick sharp sensation and probably hear a click through the PA. If it is a tingle that lasts for more than a fraction of a second then it is leakage.

This leakage is normal in a double insulated system.

The worrying thing is that your Laney monitor is supposed to be grounded which should ground the rest of the system. I'd check that the metal surround on the input jack connector is actually connected to ground.

[ef37a]

"The lack of a good connection to earth somewhere in the system would have amounted to a system in which one further fault could make the system potentially lethal?"

Definitely Ian. Our safe electrical system relies on layers of protection each one of which is a very unlikely failure scenario. Your microphones do not seem to be bonded to earth and so in the unlikely, but possible event of something going live to chassis, 'possibly' a fuse/trip will not operate.

The XLR pin one should read low resistance to mains earth (or, simply check with a DMM for mains voltage twixt p1 and earth. If those test show no continuity, make some.
That could of course result in the mother of all hum loops but if so you need to rethink and reconfigure the system.

If the XLR are bonded then the only conclusion I can come to is that the mains earth is missing or high resistance? However, if that was the case I would expect hum issues with such things as guitar amps.

Something has entered brain! Have you checked that the body of the microphone is actually bonded to pin 1? It is possible that the makers rely on the XLR shell being earthed and thus the body and grill can 'float'.

Dave.