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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Isaac Budde on February 19, 2020, 12:47:50 pm

Title: Three-pole 240V Connections
Post by: Isaac Budde on February 19, 2020, 12:47:50 pm
As a member of a cover band (who also has experience as an industrial electrician) that generally provides full-stage kit, I often see bars and other venues with three-pole 240V receptacles to which a distro must be connected. Common contenders are the 10-50R (240V hot-hot-neutral at 50A), 10-30R (240V hot-hot-neutral at 30A), and the occasional 6-30R (240V hot-hot-ground at 30A, wired with a neutral conductor from the ground pin instead of the expected ground conductor). Rarely (if ever) do we see a true 4-pole, hot-hot-neutral-ground receptacle in a bar. While I understand the safety considerations of maintaining ground and neutral isolation beyond the main panel, there often is no option to connect to power in this manner short of requesting that the venue install a new circuit with a 14-50R/14-30R/CS6369 receptacle provided. In the event that the receptacle is wired directly to the main panel where a direct, low-impedance bond between neutral and ground exists, what safety risks exist in connecting a distro with branch-circuit neutral and ground lines connected to the shared neutral? As I understand, the hot-hot-neutral connection method with internally bonded ground was legal for stovetops, ranges, and electric dryers ONLY was a legal connection method under section 250 of the NEC up until 1998. I believe that there still exists an exception under the same section for existing installations, but the exception still only applies to definite-purpose branch circuits for stovetops, ranges, and electric dryers. Thus, any connection for other purposes (such as a distro) would appear to be an NEC violation, but where do the safety issues arise? The only scenario I can think of is one in which the bonded neutral/ground connection becomes open between the receptacle and the panel, and at the same time, a fault between any one phase and any neutral/ground bonded part of the system is present.
Title: Re: Three-pole 240V Connections
Post by: Erik Jerde on February 19, 2020, 01:33:57 pm
Any significant voltage on the neutral will result in anything grounded having the same voltage.  This can range from a slight tingle to bang youíre dead.  Itís especially bad if the receptacle polarity gets reversed.  You can end up with a hot skin effect where the entire chassis is carrying mains voltage.

With your background you've probably thought of this but if the wires are run through properly installed conduit the venue owner could easily have the 3W receptacle replaces with a 4W and obtain ground via the conduit.  Wonít work for every situation of course.

Iíve often wondered how the 3W range/dryer etc connections were deemed to be safe.  I replace them whenever the opportunity arises.
Title: Re: Three-pole 240V Connections
Post by: Steve-White on February 19, 2020, 11:24:06 pm
I don't think I'd do it.
Title: Re: Three-pole 240V Connections
Post by: Brian Jojade on February 20, 2020, 12:23:40 pm
there often is no option to connect to power in this manner short of requesting that the venue install a new circuit with a 14-50R/14-30R/CS6369 receptacle provided.

Yep.  There's your answer right there.  If it's not a proper plug, it's not an option.

3 pin plugs are to be used for 240v service only. They should not be used to supply 2 legs of 110, no matter how much you really want to.  If the wiring is already pulled for it to be safe and just swapping out the plug is all that is needed, it's a trivial expense.  If new wire needs to be pulled, it can cost some $$, but it's much less expensive than the alternative.

If all you're concerned with is gear functioning, then safety ground is never actually 'needed' in a system.  Of course, without it, if anything goes wrong, well, there goes your safety.  When you get into bigger plugs, this problem doesn't go away. In fact, it's even more critical!!
Title: Re: Three-pole 240V Connections
Post by: W. Mark Hellinger on February 20, 2020, 02:06:38 pm
As Brian said above:

Isaac:  Concerning your following:
1) to which a distro must be connected.

2) there often is no option to connect to power in this manner short of requesting that the venue install a new circuit with a 14-50R/14-30R/CS6369 receptacle provided. 

3) Thus, any connection for other purposes (such as a distro) would appear to be an NEC violation,
I'll suggest there are options and plugging a distro into any 3 pole 240v outlet is an NEC violation because of the safety problems.  The options are the self evident ones you're aware of:  Pass on performing at that venue, or don't utilize a distro plugged into that 3 pole outlet... as well as the option of what I and others have done per your #2 above is talk the venue into it, or personally pay for an licensed electrician to replace/install a 14-50R where it's cost effectively feasible, and the venue's fine with it.  But that's just my opinion.
Title: Re: Three-pole 240V Connections
Post by: Jonathan Johnson on February 20, 2020, 07:31:22 pm
If you are "sharing" the neutral and ground (i.e., one conductor from the panel to the plug, and "ground" and "neutral" are bonded in an adapter plug or in the distro), there are a couple of problems.

With unbalanced loading -- for example, you're pulling 40A on one hot leg and nothing on the other -- then you're also putting that 40A load (or whatever the difference between the hot legs is) on the neutral. Because of the load on the neutral and the resistance of the wire, there is a nonzero voltage potential between the neutral/ground at the distro and the ground everywhere else. The greater the imbalance (and the greater the current on the neutral), the greater this voltage potential.

The first problem with this is one of safety: because there is a voltage potential, backed by a possibly high current source, a person who contacts both a "grounded" portion of the sound system (i.e., the shell of a microphone) and some other grounded equipment (i.e., steel strings on a guitar grounded via a backline amp plugged into another power source) may receive a shock. In a worst-case scenario, this could be fatal.

The second problem is a technical one. Because the "ground" at the distro is now contaminated with a nonzero voltage, distro-powered equipment and separately powered equipment will exhibit different potentials on their respective grounds. When you interconnect this equipment, the shield will begin conducting a current, which can be manifest as a hum in the system, especially if the interconnect is unbalanced and the shield is connected to ground.

So it's not a good idea to use these shared neutral/ground connections, even if "it's never been a problem before." Each use is a little different, and the right combination of otherwise innocent factors can turn what was once seen as harmless -- deadly.
Title: Re: Three-pole 240V Connections
Post by: Geoff Doane on February 20, 2020, 08:11:07 pm
What do the electricians here say to the idea of keeping ground and neutral separate, but running a ground wire from the distro ground to a known good ground (case of a breaker panel or similar)?  It's not a simple plug any longer, but it should be safer.  It's a similar idea to the 5-15 "cheater" plugs (which may or may not be legal).  Of course if it's like the L10-30 receptacles I picked up somewhere years ago, the neutral is bonded to the frame strap, meaning it will create a contaminated ground for anything else connected to that conduit.  Luckily, I don't see very many of these where I work.  14-50Rs are almost ubiquitous.  The only exception is the former Sheraton Hotel in town which has 18-60Rs (3 phase, neutral, no ground) in the main ballrooms.  They rewired them by connecting one of the phases to ground at the breaker panel, and then building distros to work with that.  It's not like anybody else in town has a 18-60 plug to mate with it.  ::)  Somebody specified and installed that back when the place was built in 1985.

BTW, at least according to Hubbell, "pole" refers to current carrying conductors, not the ground.  So a 14-50 is a 3-pole, 4-wire connector,  a 10-30 is 3-pole, 3-wire and a 6-30 is a 2-pole, 3-wire connector.
Title: Re: Three-pole 240V Connections
Post by: Jeff Bankston on February 20, 2020, 08:48:36 pm
This is a Cali 50amp plug. Hot , hot , neutral prongs , the metal shell is ground. I would want a separate ground prong or case ground just in case the neutral "prong" gets fried which I have seen.



 




Title: Re: Three-pole 240V Connections
Post by: Erik Jerde on February 20, 2020, 10:42:54 pm
What do the electricians here say to the idea of keeping ground and neutral separate, but running a ground wire from the distro ground to a known good ground (case of a breaker panel or similar)?  It's not a simple plug any longer, but it should be safer.  It's a similar idea to the 5-15 "cheater" plugs (which may or may not be legal).  Of course if it's like the L10-30 receptacles I picked up somewhere years ago, the neutral is bonded to the frame strap, meaning it will create a contaminated ground for anything else connected to that conduit.  Luckily, I don't see very many of these where I work.  14-50Rs are almost ubiquitous.  The only exception is the former Sheraton Hotel in town which has 18-60Rs (3 phase, neutral, no ground) in the main ballrooms.  They rewired them by connecting one of the phases to ground at the breaker panel, and then building distros to work with that.  It's not like anybody else in town has a 18-60 plug to mate with it.  ::)  Somebody specified and installed that back when the place was built in 1985.

BTW, at least according to Hubbell, "pole" refers to current carrying conductors, not the ground.  So a 14-50 is a 3-pole, 4-wire connector,  a 10-30 is 3-pole, 3-wire and a 6-30 is a 2-pole, 3-wire connector.

The problem comes in ending up with an undersized ground.  If you've got a 50A service and you run a ground bond to a 20A receptacle you're running a seriously undersized ground bond for the possible amperage it could have to shunt to ground.  That can lead to fire either on stage on in the wall.

Probably other problems too.

Title: Re: Three-pole 240V Connections
Post by: W. Mark Hellinger on February 21, 2020, 08:47:33 am
I'll offer the straight-forward sensible solution is updating the outlet to a 14-50R; however, how to get er done?  Generally taking a lead with some coercion & diplomacy is needed to move the venue to action... because "if it ain't broke, why fix it?" has likely prevailed to this point or it would already be fixed.  Likely the existing 10-50R was installed eons ago for some long-gone application (50 years or more ago?).  I've found what works to get the boat off the sand is an explanation of the seriousness of the problem & how relatively cheap & easy it is to fix, and that generally speaking, the 14-50 connection is the cheapest watts for the dollar connection available... you're proposing the most cost effective solution.
Title: Re: Three-pole 240V Connections
Post by: Stephen Swaffer on February 21, 2020, 01:04:27 pm
What do the electricians here say to the idea of keeping ground and neutral separate, but running a ground wire from the distro ground to a known good ground (case of a breaker panel or similar)?  It's not a simple plug any longer, but it should be safer.  It's a similar idea to the 5-15 "cheater" plugs (which may or may not be legal).  Of course if it's like the L10-30 receptacles I picked up somewhere years ago, the neutral is bonded to the frame strap, meaning it will create a contaminated ground for anything else connected to that conduit.  Luckily, I don't see very many of these where I work.  14-50Rs are almost ubiquitous.  The only exception is the former Sheraton Hotel in town which has 18-60Rs (3 phase, neutral, no ground) in the main ballrooms.  They rewired them by connecting one of the phases to ground at the breaker panel, and then building distros to work with that.  It's not like anybody else in town has a 18-60 plug to mate with it.  ::)  Somebody specified and installed that back when the place was built in 1985.

BTW, at least according to Hubbell, "pole" refers to current carrying conductors, not the ground.  So a 14-50 is a 3-pole, 4-wire connector,  a 10-30 is 3-pole, 3-wire and a 6-30 is a 2-pole, 3-wire connector.

Provided the grounding conductor is sufficiently sized it would work-though it probably wouldn't pass code/inspector muster because the ground can be disconnected without disconnected the power wires.  Of course, the same is true when using tails.