Lighting Instrument Repair – Grounding

Lighting Instrument Repair – Grounding

Problem defined

Grounding of professional lighting instruments is an ongoing problem that is being perpetuated with no resolution in sight. This is a real problem for a number of reasons. The first and foremost is that the manufacturer employ manufacturing methods that I believe are cost cutting and thus lead to inferior grounding integrity of most lights.

Another possible cause arises out of the history of lights and how they were used for studios. Most of the lighting fixtures used in the motion picture industry up to the 1980’s used dc power and not ac. For this reason, most lights were powered with two wire. One major lighting manufacturer representative told me that his company did not start grounding their lights until the 80’s.

There is a lot of economic pressure to manufacture instruments at a good price point to maintain market share. There are a number of manufacturers that put out a product that seems to function well at at reasonable cost. This puts pressure upon the more upscale manufacturers to follow cost cutting manufacturing and not improve safety because it is not a factor that people ask for. People shop for function, what will the light do, vs. how much will it cost. Almost no one shops for lighting instrument safety.

Faulty manufacturing methods:

Principal fault is the painting of all metal surfaces prior to putting the instrument together. The paint acts as an insulator between metal surfaces. Another incredible lapse of good safety construction is not removing the paint at the ground point. I find that the screw or rivet where the ground wire is connected measures a good ground but that connection path stops at that connector due to the painted surface.

Some lighting instruments manufacturers choose a ground point close to the light socket. This location, one could argue, is not the best location for operator safety in case an instrument becomes compromised.

Most all models of lighting instruments fail to include grounding straps from the ground point outward toward the lighting instrument shell. The grounding is adequate at some point inside the instrument but as one proceeds outward the measured resistance increases, in many cases drastically.

Cheap materials, incomplete manufacturing process, and inferior materials are used for grounding.

  • Nuts and screws are occasionally used that rust.
  • The surface which the ground lug is attached is sometimes not bare metal but some form of metal oxidation or rust is found or can develop in the future.
  • Star washers are not used when appropriate, to bite into the metal under the ground wire connector lug.
  • The use of dissimilar metals for grounding is some times used. We find aluminum against steel.
  • Pop rivets come loose for some reason. If this occurs after manufacture and sale, then the liability becomes the owner of the instrument and not that of the manufacturer.
  • Pop rivets do not have metal washers applied to the reverse side to further the metal contact of the ground.
  • The lack of grounding straps, is always missing, to extend any single point ground continuity to other parts of the instrument body.

List of problem manufacturers:

  • Feather Lights, I have had to disassemble these instruments and remove paint to allow these lights to be used.
  • Packaged Lighting has a tendency to ground near the light socket but because their are a number parts in-between the ground point and the outside case, the grounding continuity is in jeopardy getting to the outside instrument case. I have used grounding straps to extend the ground.
  • Colortran models LQBM-10, VARI 10, very old lights, failed to remove paint under the ground point. This requires instrument disassembly and sanding the point at which the instrument is grounded on both metal sides and reassembled. This takes about 20 minutes to complete per instrument.
  • Mole Richardson is one of my favorite lighting instruments due to its fine heavy duty construction and wonderful light control. This manufacturer at times paints the metal and then puts the instruments together impeding the case ground. Baby Solarspot model has a ground point at a switch box but this ground is not maintained to the body of the instrument. The manufacturer did have clean metal to metal contact from switch box to instrument but the ground point, at the switch, did have paint under the large round ground washer.
  • Junior Solarspot model also provided a painted body that failed to allow complete case ground. These instuments are rather old. I do not claim that instruments purchased after had the same problem.

Theory behind a new standard

The grounding of equipment we are trying to achieve safety for the user. We hope that if a hot wire is allowed to touch the light it will do so inside the metal instrument case. The case must be completely grounded so when the wire touches the case, the circuit breaker will trip shutting down the power. The problem with this scenario is the case is seldom at ground potential. From my experience, only one small point in the instrument is properly grounded if at all.

Using ohms law it can be calculated that a resistance of 5 or more ohms will keep the circuit breaker from tripping while the lighting instrument is at electrocution level. It is not uncommon to measure above 5 ohms on the bare metal outside points of most lighting instruments.

The factors that one uses in making an accurate calculation as to when a circuit breaker will trip are not exact. The house power fluctuates above and below the nominal expected voltage. The current at which the circuit breaker trips is not exact. We have not even calculated the IR loss between the instrument power connector and fuse panel. For these reasons this standard might change with further investigation.


There does not seem to be any standard for grounding a lighting instrument. For this lack of standard I have taken it upon myself to devise my own standard. I have set the value of one ohm as the threshold by which an instrument will either pass or fail. If I measure one or more ohms of resistance between the ground lug of the power plug and the ground point of the lighting instrument, the instrument is out of service until it is repaired. The ground point will always be a single point outside the instrument. You might be somewhat surprised at one ohm being chosen as being so high a value. Zero ohms should be the norm, the ideal. Believe me when I say that you will find maintaining that value a challenge. Expansion, contraction, corrosion, breaks in multi strand wire, all add up to increase the measured resistance over time. When you are responsible for a lot of lighting instruments, some form of compromise is called for otherwise a lot of lights will not be available.

Now here is a problem that I just can not get a grip on. Just about every lighting instrument will exhibit raised resistance the further one measures from the point the instrument is grounded. I have decided to use 3 ohms as the extreme outside measurement I will allow while taking measurements at various exposed bare metal points. I am hoping that this reading will trip a circuit breaker. I feel a bit uncomfortable using anything over 1 ohm but I just have to get real because the grounding points away from the main point is compromised by a number of factors.


Your volt ohm meter is the best tool for the detection of a ground fault. Here is a step by step method of determining if an instrument is at fault or not.

  1. Completely remove the lighting instrument from any power source.
  2. Do a complete visual inspection of the light for any and all problems.
  3. Use a volt ohm meter and select the ohms function.
  4. Choose the lowest ohm scale reading. We are going to want to measure down to one ohm or less and we want to see fractions of one ohm. I love the digital meters. Only use a meter that will give you at least three digit readout (X.XX).
  5. Short out the meter leads and write down the reading. I doubt very much if you read 0.00 ohms. You should read at least 0.01 ohms which shows you the resistance of your meter cables. You will be deducting that reading from any measurements you make on a lighting instrument, so write that number down. Also, in subsequent days, always measure the resistance of the meter leads before you measure a lighting instrument. The leads can change resistance over time showing you that you just might have to repair or replace the leads.
  6. Connect one lead of the ohm meter, it does not make any difference which lead, to the ground lug of the instrument power cable. I would hope you know which lug that is.
  7. Connect the second lead to the ground point of the light. Now this is where it gets interesting and somewhat time consuming. Where is the optimum ground point? Well I have had to take apart just about every light to find the connection point. If you measure less than one ohm at just about any outside point of the light than you are pretty sure the ground is OK. Keep in mind that most manufacturers will ground at a point not too distant from where the power cord enters the light so look for every screw, and pop rivet in that vicinity for good continuity. If you measure less than one ohm you are OK.
  8. If you measure one ohm or more you should take the instrument to the manufacturer or a repair shop that can make the necessary repair. If you know what you are doing, take the instrument apart and find the problem and make the repair.
  9. A totally separate test, but an important one, is to test for power line leakage to ground. Select high resistance setting on your meter and then measure from the ground lug on the power cord to each of the other two power connector pins. You will read NO resistance or infinity in every case. If you do not, if you read any resistance value, than you now have a serious or impending serious electrical short. Do not use that instruments until it is repaired. What you have just done is to measure the resistance between the power line used to fire the bulb and ground. A proper designed instrument will never allow any possible electron flow from a power connector and ground.


Repairs should only be made by the manufacturer or a qualified technician that fully understands the theory and practice of electricity and electrical repair. If you have any reservations at opening one of these lights then don’t. Leave the job to a qualified technician.

For the repair technician, here are some things that I have found to look out for that should help in your repair.

  • In just about every case, disassembly of the instrument is required.
  • Once you are inside the light, locate the ground wire to see how it is attached to the instrument.
  • If you do not see clean bare metal under the ground point, then assume that it was painted during manufacture and the painted surface is the reason for the ground fault.
  • You probably will have to remove the ground point for inspection. Remove the screw or drill out the pop rivet.
  • If the metal around the ground point hole is painted, sand, do not file or grind the surface. Grinding and filing will make depressions that will interfere with smooth metal to metal surface contact when you reattach the ground point.
  • When making the ground point connection think of clean metal to metal contact on BOTH sides of the metal surface when using nuts/bolts and pop rivets.
  • Do not use any connector that can slip out of place. For this reason, do not use open U shaped spade connectors. Use O shaped!
  • When attaching the ground point connection, take special care to fasten with enough force/torque to ensure future integrity.
  • The word lock washer and star washer are distinctly different during this treatment. Do not use lock washers to gain a good ground. DefinitelyDO use star washers that will penetrate existing paint at numerous points. The uglier the star looks to you and feels to the touch, the better. The star washer should have deep tits that can cut through the paint making good electrical contact with the metal surface underneath.
  • After attaching the ground point and before putting the instrument together, take a continuity meter reading again until the instrument shows a good reading. Then take measurements away from the ground point to see how bad things get. Make a decision whether further paint removal, star washers, and grounding straps are advisable for other parts of the light. Plan carefully any ground strap that comes close to the light socket. Remember that most spots can “focus” and straps must stay out of the way of the light socket movement. Use all of your common sense when doing any redesign of any lighting instrument.You might want to consult with the manufacturer.

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