| You can do it, absolutely safely, by using an isolation transformer
The transformer must have primary and secondary windings fully isolated from each other. The primary connects between Phase/Neutral of your 120V supply. The secondary powers the light
The metal chassis of the light is to be left unconnected
Since the transformer secondary lives in its own floating "potential domain", it's secondary voltage is not referenced to outside world's Earth
Any single connection between the live circuit of the secondary and the outside world : You touching live parts in the circuit or the luminaire body (with an existing isolation fault inside) etc, cannot pass current as there is no 2nd connection through the power source to complete the circuit. This is in contrast with the power supplied to your house, where Neutral of the power source is hardwired to Earth, both inside the house and at the transformer outside
Since the protection entirely depends on the non-existence of two connections between the protected area and outside world, you must take the care to prevent any possibility of 2 connections happening :
- Do not connect more than 1 electrical device to the output of the transformer
- Do not mess inside the connected light in a way, that could make you touch 2 different live parts (or a live part and the luminaire body, since it may have an existing isolation fault to another live part)
- Do not touch 2 parts of the luminaire body at the same time, if they are not reliably connected together by the luminaire construction (for example, metal ballast box and shade of a lamp, connected by non metallic body), since each part may have an isolation fault to a different live part inside
Two connections may be by you touching 2 parts, or by you touching 1 part and the other one getting Earthed by some other way (for example, by the luminaire laying on the floor or touching an Earthed metal object, or in case of audio equipment for example, by means of an audio cable connected to devices outside the protected area)
Some notices :
There is a minimal capacitive coupling between the transformer windings. That is, there will be some tiny current flowing if you touch the luminaire body (if there is an isolation fault, or if there isn't, because there is some capacitive coupling in the luminaire as well). This current will be way below any dangerous levels, allthough it may still light voltage indicators (Neon lamp or non contact type)
If you run electronics with switching power supply (such as lights with electronic ballast) from the transformer, the current coupled through the transformer's capacitance will increase, depending on HF ripple the device puts out on its Earth connection
Verify that you are using an isolation transformer with proper isolation between it's primary and secondary windings. Autotransformers, or isolation transformers with only the minimum "functional isolation", cannot be used. Choose one where the isolation is firm and reliable for use as safety isolation : The primary and secondary are wound on different layers with solid isolation (such as thick impregnated cardboard) between them, and with all necessary clearances around the edge of the isolation layer observed
Since the transformer separates the secondary from the primary circuit, any ground faults on the transformer secondary, or you getting a shock if you touch 2 live parts, won't be detected by a GFCI connected before the transformer
The transformer primary voltage is your 120V supply. Secondary can be 120V (1:1 transformer) or any other voltage as needed for your light, such as 277V. It does not matter which winding is identified as primary or secondary on the transformer itself : A transformer made for 277V to 120V conversion can be used for 120V to 277V conversion, and as long as it is an isolation transformer, it is such in both directions. Combinations of multiple transformers can be used to achieve desired voltage or power out of transformers on hand
The transformer power rating (VA) must be chosen so, it can handle your light. For typical Fluorescent or HID you can assume :
- Ballasts are 80% efficient (could be significantly less efficient if running out of spec, such as with 1 lamp on a 2 lamp ballast or with EOL lamps)
- Power factor 0.5 for LPF, 0.85 for HPF, if not stated otherwise on the ballast (HPF ballast can have low power factor if the capacitor is bad)
- Supplying a luminaire with higher voltage than rated will cause higher current by about the same margin, so higher power by the same margin squared
For example :
Old 2x90W light for 110V (which you supply with 120V - overvoltage). It seems to work ok and the ballast does not get too hot too fast, but power factor is unknown
2x90W = 180W
180W / 0.80 = 216W (0.80 = 80% efficiency)
216W / 0.5 = 432VA (0.5 power factor)
(120V / 110V)^2 = 1.19 (factor of power increase due to overvoltage)
432VA * 1.19 = 514VA
So you will need a transformer for 514VA or higher (since all figures are approximate, it won't make big difference if you take it as being 500VA)
The exact choice of transformer depends also on how long you want to run it :
For long time use, choose a bigger transformer, so it will run colder (so less stress on the isolation, which provides your safety, and generally better for the transformer)
For a quick test, a smaller transformer (such as 400VA for a not long run time, and even 250VA if its just to see that it starts) will do - since transformers have big thermal capacity, it won't heat up immediately
Since the transformer's power rating is (probably) way lower than the house circuit into which you plug it, protect the transformer with a breaker or fuse suitable for the transformer's power rating. The breaker or fuse can be installed before or after the transformer (note that different current ratings apply if the transformer changes the voltage)
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