High Intensity Discharge Lighting
IDENTIFICATION
Identification of high intensity discharge (HID) lighting which is commonly used in industrial, retail and other premises should be fairly straightforward, the high intensity of light observed normally being the main indicator. Further characteristics are a delay of several minutes in the light becoming fully operational from the time when switched on (some lamps may have a run-up time of up to 15 minutes), and a lack of being able to re-start a hot lamp until it has cooled down.
Verification of lamp type may also be obtained from spare bulbs, although care needs to be taken not to touch bulbs with bare hands to avoid possible contamination. Depending on the country or origin, Metal Halide bulbs may be denoted by the letter “M”, Mercury Vapour, “H” and Sodium “S”. Metal Halide and Mercury Vapour lamps emit a bright white or blue light, whilst Sodium lamps tend to be yellow.
Mercury vapour and sodium lamps are predominantly used for external lighting, whereas metal halide lamps are principally employed internally. Whilst some are likely to remain in service, mercury vapour lamps have tended to be phased out in the UK.
HAZARDS
All HID arc tubes deteriorate in strength over their lifetime because of various factors, such as chemical attack, thermal stress and mechanical vibration. As the lamp ages, the arc tube becomes discoloured, absorbing light and getting hotter. The tube will continue to become weaker until it eventually fails, causing the breakup of the tube.
Although such failure is associated with end of life, an arc tube can fail at any time even when new, because of unseen manufacturing faults such as microscopic cracks. However, this is quite rare.
Since a metal halide lamp contains gases at a significant high pressure, failure of the arc tube is inevitably a violent event. Fragments of arc tube are launched, at high velocity, in all directions, striking the outer bulb of the lamp with enough force to cause it to break. If the fixture has no secondary containment (i.e. an open luminaire is employed) the extremely hot pieces of debris can fall onto people and property below the light, likely resulting in injury, damage, and possibly causing a major fire if combustible material is present.
In an effort to mitigate this hazard, most manufactures produce a range of metal halide lamps which have an in-built protective shroud around the arc tube. Another manufacturer (Phillips) have produced lamps that are Teflon coated (referred to as type “P”) as protection against shattering. The general recommendation of metal halide lamp manufacturers are that lamps should always be used with enclosed luminaires, unless incorporating shroud or Teflon coating protection.
Diagram of a typical metal halide lamp with a protective shroud around the arc tube. (Despite the shroud being open at the top and bottom, manufacturers data would indicate that its presence is sufficient to protect the bulb from shattering)
Typical HID luminaires
RISK CONTROL
Risk control considerations applicable to HID lighting and, in particular, metal halide lamps, centre on ensuring clearance from combustible materials and structures, bulb containment and correct operation, care and maintenance. The following precautions should be observed:
HID lamps are to be installed and operated at all times in accordance with the manufacturer’s instructions and are to be compatible with the luminaires and fittings in which they are employed.
Lamps which are damaged or scratched are not to be used.
Lamps without shrouds or Teflon coating protection should only be employed within enclosed fittings incorporating integral tempered or borosilicate glass containment barriers.
Where situated in high risk premises such as a major high bay warehouse with a high contents fire load, a combination of both shrouded lamps and containment barriers is the preferred option.
Containment barriers must not be retrofitted to existing luminaires without the lamp and luminaire manufacturer’s approval to avoid possible compromise in safety.
All HID lamps are to be positioned clear of combustible materials and structures.
Where installed in warehouses and storage areas, HID lamps should be positioned within aisles and not directly above combustible stock.
Recommended “best practice” by a leading lamp manufacturer advocates that metal halide lamps are positioned a minimum of 2m from combustibles. However, this may result in unacceptable aisle widths, in which case distances may be lowered to a minimum of 1m, subject to containment barriers or shrouded lamps being used or, in the case of high risk premises, a combination of both.
Where HID lamps are in continuous use, they must be switched off for at least 15 minutes each week. Lamps which do not subsequently re-ignite are to be replaced immediately.
Ideally, lamps should be changed after 70% of their rated life-span. Lamp replacement should be undertaken by a competent electrician, ensuring that the correct lamp type and rating appropriate for the fitting is used. A record of lamp installation date and recommended manufacturer’s service life should be maintained.
LED CORN LAMPS
Typical LED corn lamps
Lighting equipment suppliers are marketing LED corn lamps as a direct, high-efficiency (and less hazardous) replacement for metal halide and high-pressure sodium (SON) lamps in low-bay luminaires.
In common with LED lighting generally, these corn lamps run warm, rather than hot to the touch. Thermal cooling is normally achieved by self-convection, although in the case of some higher wattage lamps, an internal fan is incorporated. Prior to purchase, assurance will be needed that the chosen LED corn lamps are compatible with the lighting installation. Also, the control gear serving existing HID lamps will need to be bypassed, which should be carried out by a qualified electrician. As with any form of lighting, it is important that an appropriate degree of clearance is provided and maintained between LED corn lamps and combustible materials.
It is anticipated that the use of HID lamps will in due course be largely phased out in favour of LED technology offering greater efficiency and safety.