Oh' this is going to take one of my special long answers to explain. It's halfway written and will follow later unless someone else short circuits the explaining before that.
Short answer, look to the NEC for all your answers. The neutral is the ribbed side. Classifications of assembly halls or use would be the next thing to look into. A stage is I believe a class 1 or 2 assembly hall proper depending upon who runs it and how large it is. (This is all from memory.) Zip cord was already explained earlier in this posting so I will spare all of you.
After that it's on to SPT-1 or 2 wire verses lamp cord. Depends upon the fixture of course but it is only permissible to be used as a direct replacement for zip cord already on the fixture - there are heat and amperage issues otherwise in addition to use. Lamp cord without checking, while similar is not the same. When possible, for stage use all equipment should be grounded and using proper types, lengths of cable and plugs.
A sconce is designed to have a bonded ground. Unless you are using 3-conductor zip cord, you won't be grounding it which violates it's design use even with any permissible exceptions for stage applications. Also since the wiring for said wall fixture will not be seen you should be complying with the NEC for permissible wires for use on stage - not just what is cheap and easy. Otherwise why not just string zip cord all over the grid? Believe me I have seen it done and also what happens when a clamp is clamped down onto it.
For a fixture, if it is zip cord, and you are given permission for it's use by the person liable for it's use and the inspector as it might also be fine, otherwise unless used in special circumstances or specific ones that are approved, follow the code.
Why do people love twist locks so much? 20 years ago they were the hot thing because they would not come apart and that was fealt safe. Now they have their uses but for the most part are not standard practice for just that reason. Unless you are using what would be a L1-15, L1-10 as they might be called or non-nema similar two prong plug use of a three pin twist lockplug would be a clear violation of code even if you yank the ground pin, just as you can't yank the U-ground part of a Edisonreceptacle. L5-15 or L5-20 plugs are grounded and you are using them on an un-grounded application and cord. That's a no no in addition to using a other than hard usage cable for the application. This is my interpitation, your person in charged might take a more leaniant stance and interpitation on such things out of necessity. Were it me, I would plug a Zip cord I needed to plug in, into a L5-15 assuming it's your plug to 5-15 adaptor, than use a polorized 1-15 plug into it. The polorized plug ensures you won't reverse hot and neutral - in other words on the screw shell of a lamp that your fingers can touch as you screw in a light bulb, it's not fed by the hot leg of power. Thus the reason for noting which part of a zip cord is hot and which is neutral also in addition to in following a standard the person that follows you hopefully won't get zapped by a accident you set up.
As for use of zip cord while working for a event company, it's common in use. As long as you don't plug in the spool and make a terrific transformer, for the most part it's safe as long as you don't overload it. The Eagle Add A Tapsystem is great. Just bought a hundred of each for a show using them. Expensive somewhat. They are now available in a polorized version which while larger I recommend so you can keep your fixtures safe to use if screw base. Otherwise without plugs and receptacles that are polorized - having a neutral prong larger than the hot, all the wiring the zip cord propery in the world won't help you get lamps pluged in properly. This is especially common when someone cuts off the wide flairing of the neutral prong from the fixture's plug so they can use it on a non-polorized receptacle. It's also a bad idea to attempt to re-use zip cord or not pay special attention to it's ends. I have seen sparks in the ceiling truss from it, question is since the breakers did not trip, just how old or warn out were the breakers, than what did it do to the wire? That's from exposed ends or ones that when they are cut are too close together than arc. It might just be zip cord but it's still dangerous without care. The add a tapsystem for the most part works well, but in piercing the jacket it does displace the wire and often damage it slightly. That damage and those holes in a wire can be dangerous to handle when the stuff is re-used with new outlets positioned without covering up the holes. One tent company tried to tell me they self seal. That would be a nope. Giving the piercing, and amperage ratings of 18/2 SPT-1, I would not put more than three amps on zip cord I was using. It's rated for up to 7 amps but would be pushing it especially once voltage drop figures in.
But lay a line of 18/2 SPT-1 along the floor, add a outlet where you have a fixture and away you go - great and fast system. Not overly safe or approved for use in all applications. Also not grounded.
There are other versions of zip cord on the market. I stock 12/3 zip cord for convention center use under carpet and with fork trucks running over it. It's also available in a actual convention center flat cable that's much similar except it does not have solid jackets over it, instead all wires are laid flat with normal insulation and a PVCjacket over them. Largest of this I have made up is a 10/5 or 6/3 version of it. Still zip cord for all intensive purposes but lots more difficult to put into a plug. Flat cable like zip cord has limited application and permissible use and is acceptable out of necessity. Some cheaper forms of office cable ramp cable also uses a three conductorzip cord with the cable ramp perminantly installed on it.
Zip cord is not hard usage cable rated, it has a lifespan of about 20 years before the rubberized jacket becomes brittle as well as similar problems when overloaded. It's voltagerating is also not high enough for low resistance wire over a long distance at a stage lighting capacity amperage. I have a piece of zip cord mounted on my wall of shame in which all you have to do is touch it and it will expose a new section of seriously corroded wire by way of the insulation just plain falling off this cable was probably subject to higher than rated amperage and the whole length of wire went bad. Not safe at all except for low amperages and special use. Tent shows and special events have their own classificaion for what is permissible to be used. You can't be plugging a grounded piece of equipment into the zip cord iether, though it's frequently done. All it takes is one well placed spilled drink and you have a lawsuit. On stage you are not even allowed to be using SJ wire in lengths over 3' and for specified purposes. Far different than rock companies get to use it for due to the fact that truss is considered adiquate support and protection for it. Now once they lay anything other than type S such as SOOW-A wire on the ground, much less as a dropline off the or between trusses, it's possible that they are also in violation. This means that the orange zip cords are also against the rules to be used especially on stage. That's a form of SJ wire.
Have I used zip cord on stage, sure. Given it's approved for special purpose use. It's about senseless to plug in un-grounded equipment with grounded heavy duty cable. It's better given such equipment is allowed if it were plugged directly into the wall but zip cord as long as used properly can be used with approval. The NEC is a set of rules and guielines but there are common sense execptions also.
It has been necessary also to put ungrounded wire into a stage pinplug. This gets even more complex because a stage pinplug can be configured to safely run two pin cable by simply removing the ground. The problem is that unless really observant it's easy to plug it in backwards. At times like this I usually cut about 1/3 of the length of the male pin off the plug leaving a sanded to finished nub stickign out where the ground was. This confirms that the cable is not grounded as necessary I would interpite, plus helps to ensure the cable will not be plugged in backwards. On the other hand it's against the NEC because in doing so it is modifying the plug. In the case of a stage pinplug the exception of leaving the ground in place I would say is permissible and that's about the only cable for me I allow. No grounded Edison plugs or twist receptacles on circuits that are not grounded. Just one of quarks.
There it's un spell checked but done plus a bit shorter than it will have been.
I inherited the theatre I am TD of in pretty bad shape, I am trying to figure out why things were done in certain ways I had never seen before. Wait until I post a picture of our sound racks. One day I was hanging some new fixtures in the catwalk when one of the worklights shorted out and actually started a fire on a piece of tie-line. Luckly I was there to stamp it out. I found it was a Par 56 with a 500w lamp being used as the work light and it was wired to a edisonplug with zip cord. Exactly as you said a c-clamp had torn into it and shorted it out.
Guess I need to get out he NECbook and find out why a convention space is in a different category than a theatre.
Specifically how would you wire a lamp socket? Would you use a grounded cable? If you so how would you terminate the groundwire at the socket?
A 500w lamp is 4.16 amps. Given the zip cord is 18/2 it could safely run it for a year or three before I would downrate it's insulation to good for 3 amps. But 4 amps is not that huge a load unless it is continuous duty. This type of cable however is home owner grade and not acceptable for your use. Nor is it rated for lamp cord given the wattage and temperature. I'm sure if you stuck your hand into the lamp cap and bent the wire it's insulation would just flake off revealing corroded wire. How was it spliced to the lamp cap anyway wire nuts? I see this all the time. Job security.
How I would wire a lamp socket depends upon the socket. I count 139 types of lamp base currently in use with multiple designs each has for wiring to it.
In the case of a PAR 56, it uses a EMEP or GX-16D lamp base the same as a PAR 64 would. For a ground on a PAR can, it's standard that the groundwire is attached to the cap/top of the can. I'm a big fan of terminating the wire with a #10 stud ring terminal and attaching it with a actual electrical grounding screw - the green ones with the slotted hex heads that are thread forming. A washer and a stainless steel 10-32 top lock nut on the inside of the can. There is no way this screw is coming loose unless someone installed the nut backwards in which case it will strip itself.
That's as opposed to using a normal zinc screw that can rust, and a nylon lock nut (nylock) that if it gets warm enough the nylon will melt away from the screw making it just a normal no locking nut. Much less any crownnut or nut with lock washer that will not stick around to much abuse. Rivets are also suspect because they come loose. Always check your groundwire is not loose when inspecting the can. It's the difference between getting shocked and not.
As for the socket, I use my EMEP sockets made by Sylvania. Good wireing and a aluminum frame around the part that usually breaks on the socket. In other words it will survive flopping around inside the can without breaking.
One good test on all EMEP sockets is a tension test. If it will pick up a lamp than it's the proper tension. If it won't than it is trash or at least it's porcelain is spare parts. Don't try to re-tension the springyness of the PAR contacts. Yea you can do that but it won't last very long and can break especially once the current starts going thru it.
On the screws holding together the old porcelains, high temperature threadlocker works well as with replacing the nut with a real nut. Or in general when prepping the can, just take a screw driver and ensure the screws holding together the lamp base are tight. Something else you don't have to worry about on a more modern lamp base.
For wire, I use type K, (FEP) 150c in a 16AWG for the ground. It's not rated for as high of a temperature but I usually prefer my ground wires rated for less in temperature than my current carrying conductors. This way when you see a melted groundwire, it's probably also about time to swap out the other wiring too.
Lamp bases come with Type SF heat wire rated at 200c. For a PAR can that's fine. If you find that the fiberglass is wearing off a bit by the lamp base but the conductors are not exposed you can install some better quality fiberglass spegatti tubing over it or use the silicone coated fiberglass. That's one advantage the old lamp bases have in that you can put the tubing inside the lamp base. Another note about the old lamp bases is that it's often a good thing to tie the power cords coming off the lamp base into a square knot directly behind the lamp base. This will pick up a bit of the strain of the wire from the tugging and twisting that the wire otherwise is having done to it otherwise directly in the lamp base. Frequently this is the place that wears out fastest due to both heat and movement.
I'm a huge not fan of the heycostrain relief. Rubber of that slender thickness does not have a part inside an oven. Much less plastic. I prefer either a weather tight or two screw conduitstrain relief. Say, am I sounding like a broken record? Could sware I have said all of this before.
Anyway, at the strain relief I use fiberglass high temperature electrical tape. This disperses the pressure on the wires and fiberglass sleeving and prevents it from cutting into the above. It also prevents the wire from bending too tightly at the strain relief another cause of it's failure.
At the plug, after the company heat shrink with clearheat shrink over it is installed on the wire, I do two layers of friction tape directly to the wire than slide the fiberglass over it. About 2" in length At work for the tech people we stop at that now. This will help to grip the fiberglass and prevent it from slipping out of the plug's strain relief and prevent the cord from bending too tight and protect the wires from that stress.
It's better if you don't cut the friction tape after you slide the fiberglass sleeve over it and instad continue wrapping the sleeving above the fiberglass with two more layers. about 1.1/2" in length. This way the fiberglass is sandwiched and cushioned inside the tape, and protected against the clamp cutting into the jacket. Most don't like friction tape use at all. "it's gummy when it's warm" "it gets dirty" "It's 2004, what are you doing using friction tape?" and many more things. In fact I'm the only one using it on plugs that I know of. To shut them up I kept the friction tape to the underside of the fiberglass only.
Still, take a Bates plug - modern stagepinplug and install the cord and strain relief in the standard flat/rounded way, than try the friction tape with a round/round way or round/flat way depending upon the thickness. You will find with a tug that the fiberglass sleeving does not slide out of a friction taped connection but will otherwise without it. This saves money on materials and repair labor.
After all of this there is high temperature wire splicing methods that are safe and ok to be using inside a lamp cap but I think that's enough for one night. When your wire wears out but the lamp base is good contact me.
If you are grounding the fixture than that ground goes to the frame not the lamp base at all. The idea is that if anything shorts or breaks loose, it does not create the path of least resistance that would be thru you upon touching it. Otherwise if not grounded on a medium screw lamp base (E-26 for those interested in the US, E-27 in Europe) you tend to always want to use a Nema 1-15 plug (US again) that's polorized. This way you insure that the neutral is going to the outer shell of the screw base. Now I know I'm a broken record - oops CD.
I guess it depends upon the fixture. A little desk lamp prop on a table is probably not worth it as long as the plug is polorized and in electrically safe condition - that is unless the fixture is going to be on hour after hour in a professional setting. Plus if it's a plastic fixture, than there is not much to ground to that will be of use. Some such fixtures will have the ground even on a plastic fixture going to the conduit in the gooseneck if it's metal. But if for instance on a Ghost light that is left on 24hours a day at times, and it's expected to take a lot of abuse, than grounding it would not be a bad thing. Clip lights used during shows, them little store bought reflectors, not worth it. Etc. you get the idea. Stage lighting proper is stuff that should be grounded for sure. After that, it's a judgement call. If the fixture was once or is designed to be grounded than it needs to remain that way like for instance on a wall sconce.
Overall it's not wise to ground the screw shell of a lamp base nor attempt to ground to the frame of a cop gumball light or Mars Light as we call them. Both are the neutral. You do want to ensure both are insulated from metal or the frame or any voltage on the neutral will also be in anything you touch. The neutral while in geneal safe can carry a voltage. Treat it as a hot for safety concerns.
Fluorescent lamps also have advantages to being grounded in a different way. It helps them start in many cases.