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Incorrect power wiring to QSC K12

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Incorrect power wiring to QSC K12

Postby gogi » Sun Mar 18, 2018 9:53 am

Hello all. Just sharing a recent scenario where an incorrect (reverse polarity?) wired power plug resulted thankfully in the K12 simply switching itself off, rather than cause some damage.

I rented out several K12s to a client and he called to say one simply had its power light go off as soon as power was plugged. I had gasped and assumed something terrible had happened at worst; or at best a fuse needed to be replaced. I asked them to put it aside awaiting when I get it back for troubleshooting.

To summarize, it turned out that the power cable was defective. The 13A plug had white/black/green wires. The green wire was not even terminated to the earth on the plug. The other end that connects to speaker itself was factory sealed. I used a meter and discovered that the neutral and the Live were simply mixed up at both ends. Meaning the neutral at the 13A plug was continuous with the live at the other end. Similarly, the Live at the plug end was continuous with the neutral at the other end. So definitely incorrect wiring!


Now I know for a start that there was no earth connection at the plug end which is not good at all. But I also thought that as long as there is a 240v differential between the neutral and Live, even if they are swapped, the circuit should be continuous and deemed closed..Basically the cable was a two wire cable, no earth.

I have rectified that particular incorrect (swapped neutral/live at the plug end) wiring, and terminated the earth wire. As soon as I fixed the cable, all was fine with the K12.

So I'd like to get some comments:
1. Please confirm the K12 has some inbuilt protection to shutdown power once it senses incorrect wiring and/or floating earth.

2. Assuming a two wire power supply is used (no earth); but correctly terminated at both ends with the appropriate Live and neutral terminated, will the K12 have any issues?

PS: I am not advocating not connecting the earth. That is a no-no. Just running this theoretical scenario and doing a 'what-if analysis' for academic purposes .
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Re: Incorrect power wiring to QSC K12

Postby Wonks » Sun Mar 18, 2018 10:33 am

I'd think the answer to 1 can only really be obtained from QSC themselves.

An earth/ground was probably being provided by the XLR lead back to the mixer, which would still give enough of a ground reference to allow any fault sensing circuit (if it does exist, which I suspect must be the case) to operate. Otherwise there shouldn't have been anything stopping the power supply from working with 240v but no ground

White and black are US wiring colours, so that cable really shouldn't have been used or supplied in the UK/Europe. Was this a power lead that came with the speaker or was it one that has been made up by yourself or someone else?
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Re: Incorrect power wiring to QSC K12

Postby gogi » Sun Mar 18, 2018 10:37 am

Cable originated from US; and (incorrectly) terminated by someone after replacing the US plughead with a regular U.K. 13 amp plug
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Re: Incorrect power wiring to QSC K12

Postby Wonks » Sun Mar 18, 2018 10:53 am

Had the cable been PAT tested?
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Re: Incorrect power wiring to QSC K12

Postby gogi » Sun Mar 18, 2018 11:21 am

Short answer is no! As stated I simply used a multimeter to do a quick polarity/earth continuity check. I also know that reverse polarity can result in a fuse not blowing since it may theoretically be inline with the neutral and not the live. Also a reverse polarity plug may theoretically result in device always being live because the switch will effectively (incorrectly ) be on the neutral leg.
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Re: Incorrect power wiring to QSC K12

Postby Hugh Robjohns » Sun Mar 18, 2018 11:56 am

gogi wrote:I used a meter and discovered that the neutral and the Live were simply mixed up at both ends. Meaning the neutral at the 13A plug was continuous with the live at the other end. Similarly, the Live at the plug end was continuous with the neutral at the other end. So definitely incorrect wiring!

You haven't stated which colours you've assumed to be which, so for the avoidance of confusion to others, this is an American mains flex colour code in which the Line supply is on the black wire, the Neutral is on the white wire, and the safety earth is on the green.

Now I know for a start that there was no earth connection at the plug end which is not good at all.But I also thought that as long as there is a 240v differential between the neutral and Live, even if they are swapped, the circuit should be continuous and deemed closed. Basically the cable was a two wire cable, no earth.

Yes. But without the earth connection perhaps the protection system got upset at switch-on.

2. Assuming a two wire power supply is used (no earth); but correctly terminated at both ends with the appropriate Live and neutral terminated, will the K12 have any issues?

It's a question you need to ask QSC, but quite possibly, yes! With no ground reference who knows what the circuitry might do?

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Re: Incorrect power wiring to QSC K12

Postby Hugh Robjohns » Sun Mar 18, 2018 12:03 pm

gogi wrote:I also know that reverse polarity can result in a fuse not blowing since it may theoretically be inline with the neutral and not the live.

Incorrect wiring is never acceptable, obviously, but the fuse not blowing is only an issue in a specific fault event -- a catastrophic failure in which the fuse is in the neutral side and the equipment's internal mains wiring becomes short to the case (etc). In most common over-current fault conditions the equipment's mains fuse would still blow even if in the neutral path.

Also a reverse polarity plug may theoretically result in device always being live because the switch will effectively (incorrectly ) be on the neutral leg.

Again, yes in that the internal wiring would remain live, but the equipment won't actually be powered and operational except in the case of a significant internal fault.

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Re: Incorrect power wiring to QSC K12

Postby wireman » Sun Mar 18, 2018 12:12 pm

Unless it is double insulated (it would have a symbol to show this) you need an earth.
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Re: Incorrect power wiring to QSC K12

Postby James Perrett » Sun Mar 18, 2018 3:35 pm

If you are doing this as a business then you need to ensure that your kit complies with all the relevant regulations. That cable should have been thrown away and the correct UK cable used. The correct cables aren't exactly expensive.
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Re: Incorrect power wiring to QSC K12

Postby MarkPAman » Sun Mar 18, 2018 4:50 pm

Is it not the case that Schuko plugs, used in most of the rest of Europe, a/ have no fuse and b/ can be inserted either way around, so the distinction between "live" and "neutral" does not matter.
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Re: Incorrect power wiring to QSC K12

Postby Hugh Robjohns » Sun Mar 18, 2018 5:42 pm

It is, yes. But in Europe they have relied on double-pole breakers for domestic power circuits rather than fuses for a very long time.
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Re: Incorrect power wiring to QSC K12

Postby wireman » Sun Mar 18, 2018 9:18 pm

MarkPAman wrote:Is it not the case that Schuko plugs, used in most of the rest of Europe, a/ have no fuse and b/ can be inserted either way around, so the distinction between "live" and "neutral" does not matter.

It may not matter in "the rest of Europe" because of alternative ways to provide protection but you can't assume it does not matter in the UK. As for fuses, this is imporant and hence it is a legal requirement that any electrical products supplied to consumers in the UK market have a BS1363 plug with a fuse.
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Re: Incorrect power wiring to QSC K12

Postby Hugh Robjohns » Mon Mar 19, 2018 10:40 am

James Perrett wrote:If you are doing this as a business then you need to ensure that your kit complies with all the relevant regulations. That cable should have been thrown away and the correct UK cable used. The correct cables aren't exactly expensive.

This is a good point and worth raising for the benefit of others: if hiring kit out there is an implied duty of care to make sure all the provided equipment and mains cables (etc) are safe -- and that generally means performing a PAT test on everything before it leaves the store cupboard.

Had this situation taken a different turn with its absent safety earth and mis-wired cable the OP could be in a very different and far less pleasant position now... :o

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Re: Incorrect power wiring to QSC K12

Postby MarkPAman » Mon Mar 19, 2018 6:27 pm

wireman wrote:It may not matter in "the rest of Europe" because of alternative ways to provide protection but you can't assume it does not matter in the UK.

Sorry, the intention of my post was not clear at all, was it?

Of course it matters from a safety point of view, in the UK, as it's the way things are set up over here. Arguably, a safer system - certainly out plugs & sockets are better (until somebody puts one of the stupid "child proof" things in the socket and bypasses at least two of the built in safety features http://www.fatallyflawed.org.uk).

However, it probably does not matter, as far as the operation of the equipment goes. As it can easily be connected either way on the continent, with no way of knowing which wire is nominally 230V and which is (lower)V, it needs to be able to work safely, whichever way round it is.

So, the equipment that was not working in the OP, is almost certainly not due to the two live conductors being swapped.
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Re: Incorrect power wiring to QSC K12

Postby wireman » Mon Mar 19, 2018 8:04 pm

MarkPAman wrote:...
So, the equipment that was not working in the OP, is almost certainly not due to the two live conductors being swapped.

Yes, it would seem unlikely.
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