When a manufacturer produces an item of explosion protected equipment, it must be tested and inspected to ensure that it complies with the required standards relating to that type of protection.
All electrical apparatus and associated circuits are required to be tested periodically in accordance with a definite testing routine with recorded test results.
Insulation resistance, earth loop resistance and earth continuity resistance tests are required to be made, the last two in relation to the setting or rating of the protective devices associated with the apparatus and its circuitry.
It is important that insulation resistance tests are NOT made in such a way that the safety devices and insulation used in intrinsically safe apparatus and circuits are damaged by excess test voltages.
No apparatus should be opened in a danger area until it has been made dead and effective measures (e.g. locking-off the isolating switch) have been taken to prevent its being made live again inadvertently.
Where, for the purpose of electrical testing, it is necessary to restore the power supply before the apparatus is reassembled, tests should be made using a suitable gas detector and continued during the operation to ensure that the combustible does not approach the explosive limit. Unless the hazardous area can be made gas-free or otherwise safe, or the electrical equipment is removed from the area, then insulation resistance testing should be carried out using a 500 V d.c. tester of certified intrinsically safe (Exi) design.
The testing and maintenance of flameprooj or intrinsically safe equipment should be entrusted only to competent persons who have received instruction in the special techniques involved. The body material of instruments and tools required for maintenance purposes should be designed so that they will not make a hot spark when dropped.
The energy output of all intrinsically safe instruments should be so small that they do not produce hot sparks. An insulation tester has a drooping characteristic to prevent high currents and may be intrinsically safe when applied to circuits of small inductance or capacitance but a risk may arise when such energy-storing properties of a circuit have an appreciable value.
Where such instruments are used the ( test leads should be firmly connected throughout and on completion of the test they should not be detached until the circuit has been discharged through the testing instrument (leave the tester for one minute after test is finished).
The previous sections covering zoning, gas grouping, temperature classification and the various types of protection methods show that the design of electrical equipment for hazardous areas is very special.
Maintenance of such apparatus must not, in any way, cause its operation to be less safe than in its original certified state. This most important point means that the maintenance must be carried out by a competent person. Temporary lash-ups, refitting with wrong sized components (e.g. lamps), failing to employ the correct number of cover bolts etc., is absolutely forbidden.
The inspection and maintenance of Exd (flameproof) enclosures for luminaires, switches, junction boxes, push-buttons, etc., requires meticulous care. The following example gives a guide to the inspection and maintenance points as applied to a flameproof luminaire:
This will reduce the enclosure strength. To ascertain the extent of corrosion, remove dirt, loose paint and surface corrosion with a wire brush. If only the paintwork is deteriorating, the enclosure should be repainted to prevent further corrosion.
Make sure that there are no missing bolts. This is particularly important on flameproof luminaires because a missing bolt will invalidate the certification. Replacement bolts must be of equivalent strength as originals (usually high tensile steel).
Ensure all mountings are secure. Corrosion and vibration are severe on ships and can cause premature failure.
Examine the flamepath for signs of corrosion or pitting. If the flamepath needs cleaning, this should be done with a non- metallic scraper and/or a suitable non- corrosive cleaning fluid.
Examine the cement used around lamp- glass assemblies both inside and outside. If the cement is eroded, softened or damaged in any way, advice should be sought from the manufacturer regarding repair. If deterioration of the cement has occurred, a complete new lampglass assembly should be fitted.
Check lampglass; if cracked or broken a complete new lampglass assembly should be fitted. Clean the lampglass.
When re-assembling an Exd enclosure you must ensure that the following points are covered: Lightly grease all flamepaths and threaded components with an approved form of non- setting silicone grease. Care must be taken to ensure that blind tapped holes are free from accumulated dirt or excessive grease which can prevent the correct closure of flamepaths, or cause damage to the tapped components. Fit new lamp of the correct rating.
Ensure bolts are not over-tightened as this can distort flamepaths, cause excessive stress on lampglasses or distort weather proofing gaskets, if fitted, allowing the ingress of liquids and dusts. Check the luminaire is installed in accordance with the requirements of the installation, particularly the classification of the area if it is hazardous and that the correct rating of lamp is fitted. Remove any build-up of dust on the luminaire, this can cause overheating as well as acting as a corrosive agent.
Before attempting any maintenance work on Exd equipment check for any particular inspection and overhaul instructions given by the manufacturer.