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Author Topic: Electrical Safety Advice Needed  (Read 13654 times)

Gary Christenot

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Electrical Safety Advice Needed
« on: January 03, 2016, 08:00:38 AM »

My son is building this project (see attached photo) for a high school band.  He had the case constructed by Olympic Cases and is populating the electronics.  When I looked at it I had some concerns about electrical safety and need some advice.

Power in will be via the blue Powercon connector at the lower right, then to a rack-mounted power supply/conditioner which is in the rack right below the drawer.  Power out to the speakers will be via the grey Powercon to the left of that (one on each side of the cart).  Here are my concerns and questions and would appreciate some knowledgable advice.

1.  The Powercon connectors and XLR jacks passing through the steel plates at the bottom of each side of the cart are pretty exposed in the interior carcass of the cart.  To deal with this, we are building a couple of wooden enclosures that will go over the rear of the connectors on each side of the cart, creating a separate sub compartment to cover and protect them.  Those enclosures will be fastened inside the cart with a couple of L-brackets so they can't be moved or dislodged without intention.  We will also be adding Neutrik SCDR boots to the rear of the Powercon connectors.

2.  How to make the transition from the Powercon connectors to a useable physical interface for the rack mounted power supply and speaker out power?  Wire to a receptical in a box?  Pigtail to a 3-prong plug?  With of course female fixtures for incoming power and male for outgoing.

3.  Grounding of pass through plates.  I know just enough about electricity to be dangerous but I do know that the plates that the Powercon and XLR adapters are passing through need to be grounded for safety. How to do that? Would I be correct that the rack rail will most likely be grounded through the case of the power supply via the case mounting ears, and the power supply is in turn grounded to the main.  So my thought is to mount a grounding wire to each pass through plate and then to one of the rack rails.  Or alternatively, insert a junction box between the Powercon incoming connector and rack power supply and ground the case fixtures to the main ground there.  Thoughts?  Problems?

4.  Grounding of other case elements.  With the exception of the side-mounted handles I think all of the aluminum and steel forming the edges and corners of the case is in continuous contact with itself all the way around. I'll need to check and verify that.   I assume all of this exposed metal, including the handles, also needs to be grounded for safety.  Connect this to the same ground point as the pass through plates?

5. Verify and test for safety?  My biggest concern is the possibility, however remote, of something in the case shorting to a piece of metal on the cart and creating an electrical hazard because of insufficient or improper grounding.  How to verify all is safe?

Thanks!
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Jordan Wolf

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Re: Electrical Safety Advice Needed
« Reply #1 on: January 03, 2016, 02:03:25 PM »

All of the gear in the rack should have the chassis internally connected to the electrical safety ground. Any fault should cause a safety measure to trip - blown fuse, breaker, etc.

Neutrik sells boots that slide over the exposed contacts of the Powercon connectors, or there are various metal enclosures that will suffice.


- Jordan Wolf
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Jordan Wolf
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Stephen Swaffer

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Re: Electrical Safety Advice Needed
« Reply #2 on: January 03, 2016, 02:39:26 PM »

I would agree that all metal parts need to be "bonded" together.  Usually this entails an intentional electrical connection with a jumper wire.  I would not rely on two painted metal surfaces being in contact as my bond.  If you remove the paint where a screw/bolt is connecting two pieces that should suffice.

The second question that pops into my mind is, is the powercon inlet connector adequate for the load you intend to supply?  Most non-electricians are more concerned if power is 120 or 220, etc-but the biggest variable we run into is the ampacity of the load.  I am not very familiar with powercon ratings, but following discussions on here it seems that you may be limited to 15 amps.  Depending on the number and size of the powered speakers you intend to supply, you could quickly reach this capacity.

I hope this statement "With of course female fixtures for incoming power and male for outgoing.", is a mis-type.  Normally, you use female sockets to supply power (outgoing) and male 'inlets' for incoming power.  In any case, I would think it easiest to install some receptacles inside the case supplied by the powercon.  As Jordan mentioned, this really should be fused or breakered locally-sized to protect the internal wiring you install.
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Lyle Williams

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Re: Electrical Safety Advice Needed
« Reply #3 on: January 03, 2016, 03:33:17 PM »

If I was safety "testing and tagging" such and item under portable appliance test standards here in Australia, I would:

Physically check for solid construction and no exposed conductors.  Check that the connectors are suitable for the labelled load rating.

Check the polarity and continuity of each electrical connection.  If the constructor is doing the testing it is important to ensure that a misunderstanding during construction isn't carried over to this testing.

Confirm that there is continuity between the earth pin and significant exposed metalwork.  Some common sense is required here.  We're looking for anything that could be energised by a loose wire etc.  If the case is operated with the front/back off, the assessment of "exposed metalwork" should be done on that basis.

Confirm the integrity of wiring insulation with a megger.
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John Roberts {JR}

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Re: Electrical Safety Advice Needed
« Reply #4 on: January 03, 2016, 04:35:49 PM »

If any of the rack mount products have 3 wire plugs, the metal should already be grounded, unless you used insulating screws... The insulating screws were used to reduce pin 1 problems in old legacy (read poorly designed) gear.

JR
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Tim McCulloch

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Re: Electrical Safety Advice Needed
« Reply #5 on: January 03, 2016, 05:06:06 PM »

1.  The AC service needs a back-box over the exposed terminals and connections (the boots are not sufficient by themselves), and this box must be of non-combustible material; next you cannot mix different *services* in the same box.  The AC PowerCons need to be on one plate/same back box and the audio on the other plate, which does not require a back box but I'd recommend a separate box anyway, for protection of the terminations.

2.  With a bigger back box, you can install a GFCI outlet and use a power strip (without surge protection) for the items in the cart.

3.  The metal plate with the now-colocated PowerCons (see #1) must be bonded to the Equipment Grounding Conductor.  If your back box is plastic, it will not/cannot be bonded.  If it's metal it must be bonded to the EGC.

4 & 5.  I've not used or handled any similar case that has the metal trim grounded separately.  If you use a metal back box it should not touch any other metal object; obviously not a problem for plastic boxes.  If you use a GFCI (#2) any fault with a device plugged into the power strip should trip the GFCI and de-energize the entire assembly.  Also from the picture it looks like the rack rails and equipment do not touch the trim, so a failure that would energize the rails prior to the GFCI trip would not energize the case trim.
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Craig Hauber

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Re: Electrical Safety Advice Needed
« Reply #6 on: January 03, 2016, 06:49:17 PM »

1.  The AC service needs a back-box over the exposed terminals and connections (the boots are not sufficient by themselves), and this box must be of non-combustible material; next you cannot mix different *services* in the same box.  The AC PowerCons need to be on one plate/same back box and the audio on the other plate, which does not require a back box but I'd recommend a separate box anyway, for protection of the terminations.

2.  With a bigger back box, you can install a GFCI outlet and use a power strip (without surge protection) for the items in the cart.

3.  The metal plate with the now-colocated PowerCons (see #1) must be bonded to the Equipment Grounding Conductor.  If your back box is plastic, it will not/cannot be bonded.  If it's metal it must be bonded to the EGC.

4 & 5.  I've not used or handled any similar case that has the metal trim grounded separately.  If you use a metal back box it should not touch any other metal object; obviously not a problem for plastic boxes.  If you use a GFCI (#2) any fault with a device plugged into the power strip should trip the GFCI and de-energize the entire assembly.  Also from the picture it looks like the rack rails and equipment do not touch the trim, so a failure that would energize the rails prior to the GFCI trip would not energize the case trim.

One rule of thumb I always try to adhere to is to separate high voltage and low voltage in separate enclosures -so if my eyes are seeing the picture properly I wouldn't put XLR's in the same dish as power.

When I was building cases I ended up by installing a foot-square (or bigger) flush door/hatch in the side or back of the case that when you opened revealed all of your connections on a recessed panel inside.  In that panel I would mount old-work metal boxes for the high voltage stuff which either would internally be connected to surface boxes with raised cover mounted receptacles using emt, m-c, or chase nipples.  All the low-voltage stuff would just be banded looms with exposed connections to XLR's speakons or whatever. 

I ended up going this route because inspectors like seeing things they know -and properly installed nema generic parts always yielded the best responses compared to the arguments we would get with "mysterious" stuff like powercons mounted on stamped jack dishes and soldered to looms behind.  (I still shy-away from powercons due to past experiences with that (-and the electrician in me still likes my Hubbell twistlocks!)

There would be no specific grounding of the case metal trim as most of my cases were of the plywood type with either rat-fur or case-over type shock racks
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Craig Hauber
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Gary Christenot

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Re: Electrical Safety Advice Needed
« Reply #7 on: January 03, 2016, 08:31:12 PM »

If I was safety "testing and tagging" such and item under portable appliance test standards here in Australia, I would:

Physically check for solid construction and no exposed conductors.  Check that the connectors are suitable for the labelled load rating.

Check the polarity and continuity of each electrical connection.  If the constructor is doing the testing it is important to ensure that a misunderstanding during construction isn't carried over to this testing.

Confirm that there is continuity between the earth pin and significant exposed metalwork.  Some common sense is required here.  We're looking for anything that could be energised by a loose wire etc.  If the case is operated with the front/back off, the assessment of "exposed metalwork" should be done on that basis.

Confirm the integrity of wiring insulation with a megger.

Thanks Lyle.

The case was built by Olympic Cases and is built like a tank.  We will take road worthiness as a given.

Cheers,

Gary

http://www.olycase.com/

However they only built the physical enclosure and aren't dealing with any of the installed equipment or electricity
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Gary Christenot

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Re: Electrical Safety Advice Needed
« Reply #8 on: January 03, 2016, 08:42:02 PM »

I would agree that all metal parts need to be "bonded" together.  Usually this entails an intentional electrical connection with a jumper wire.  I would not rely on two painted metal surfaces being in contact as my bond.  If you remove the paint where a screw/bolt is connecting two pieces that should suffice.

The second question that pops into my mind is, is the powercon inlet connector adequate for the load you intend to supply?  Most non-electricians are more concerned if power is 120 or 220, etc-but the biggest variable we run into is the ampacity of the load.  I am not very familiar with powercon ratings, but following discussions on here it seems that you may be limited to 15 amps.  Depending on the number and size of the powered speakers you intend to supply, you could quickly reach this capacity.

I hope this statement "With of course female fixtures for incoming power and male for outgoing.", is a mis-type.  Normally, you use female sockets to supply power (outgoing) and male 'inlets' for incoming power.  In any case, I would think it easiest to install some receptacles inside the case supplied by the powercon.  As Jordan mentioned, this really should be fused or breakered locally-sized to protect the internal wiring you install.

The Powercon connector is rated at 20 amps.  However the rack mounted power supply is only rated at 15 amps.  According to the specs, power consumption on the speakers is 2 amps each and there are 3 of them - 2 speakers plus a sub woofer.  They QSC K12 speakers and QSC Ksub woofer.  So a total of 6 amps for the speakers.  Not much else in the system of consequence.  The mixer is a Behlinger - 60 watts consumption I think.  The fans are negligible.  The keyboard is powered off USB from the laptop so not much draw there.  I expect the small monitor speaker facing the player is also low consumption.

Gary
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Gary Christenot

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Re: Electrical Safety Advice Needed
« Reply #9 on: January 03, 2016, 08:44:28 PM »

If any of the rack mount products have 3 wire plugs, the metal should already be grounded, unless you used insulating screws... The insulating screws were used to reduce pin 1 problems in old legacy (read poorly designed) gear.

JR

Every thing in the rack is 3-prong and all devices will get power from a rack mounted power supply that is also 3 prong.
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ProSoundWeb Community

Re: Electrical Safety Advice Needed
« Reply #9 on: January 03, 2016, 08:44:28 PM »


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