Inspection and maintenance
All equipment installed in hazardous areas and all parts of intrinsically safe, or similar, installations require periodic inspection to verify the continued integrity of the methods of protection. It is possible for installations to become unsafe due to deterioration, damage or alteration while the installation remains functional. A programme of initial and periodic inspections and, if necessary, remedial action is recommended in BS EN 60079-17 (reference 15S).
Inspections are generally initial, periodic or sample. An initial inspection is recom- mended before a new installation or re-installed equipment is brought into service. The objective is to verify that the equipment is suitable for its location, that it has been correctly installed and is in good condition. A periodic inspection is intended to check that the equipment continues to be suitable and a sample inspection can be used to check the effectiveness of the inspection and maintenance arrangements.
Inspections can also be visual, close or detailed. A visual inspection involves no more than an examination of the exterior of the equipment and it does not require the use of tools or the isolation of equipment for electrical safety. A close inspection may require access equipment and basic tools but does not involve opening of equipment enclosures or isolation for electrical safety. A detailed inspection may be carried out in the field or the equipment may be removed to a workshop; isolation, opening of the enclosure, and a wider range of tools are required, and test equipment may also be used in a hazardous area if suitably protected, or in a workshop.
The inspection strategy is designed for optimum use of resources. At first sight, zero defects is a desirable target. If this is achieved there is no way to determine if the grade and frequency of inspection are excessive. Too many defects indicate that the inspec- tions are not sufficiently frequent or thorough. The strategy, therefore, compares the current results with the previous results and adjusts the frequency and/or grade of inspections until a small number of minor defects are found at each inspection. Records need not be kept but may be useful in identifying trends.
The BS EN 60079-17 contains sample inspection schedules for the various methods of protection, depending on whether the inspections are initial or periodic. It may be desirable to include work instructions, access requirements and recommended tools in the inspection schedules. A means of prioritizing and recording the findings can also be included. It is important to produce schedules which can be practically used in a plant environment. The schedules may also incorporate electrical testing and any statu- tory requirements.
Withdrawal from service
When equipment is removed from the site, the exposed conductors should be terminated in a suitable enclosure, or isolated from all sources of supply, insulated and protected from the environment, or isolated from all sources of supply, earthed and protected from the environment.
Intrinsically safe installations do not normally need to be isolated to protect personnel from electric shock or to prevent ignition-capable sparking. Consequently, the following maintenance activities are permissible in a hazardous area without isolation:
● disconnection, removal or replacement of any item of equipment or cabling in an intrinsically safe circuit
● adjustment of calibration controls
● removal or replacement of plug-in assemblies or components
● use of test equipment or procedures specified in the system documentation
Safety earths must not be disconnected or tested while the associated intrinsically safe circuits are connected into the hazardous area.
All other installations, except type of protection nL, should be isolated, including the neutral, before work commences. The documentation supplied with the equipment, or the certificate, should be consulted for isolation requirements for type of protection nL.
Fault loop impedance or earth resistance
The standard types of test equipment for these measurements uses voltages and currents which are ignition-capable and should not, therefore, be used in a hazardous area without an appropriate permit to work. They may also cause ignition-capable currents to flow in a hazardous area even when used in a non-hazardous area, espe- cially if there are faults in the earthing and bonding system. It is preferable to carry out such checks before flammable materials are brought into a plant or during a major shutdown when all flammable materials have been removed from the plant. If such measurements are duplicated by measurements using intrinsically safe test meters, comparisons can be carried out in the intervening period. If there is then evidence that the earthing and bonding system is deteriorating, it can be confirmed using standard test equipment with suitable precautions.
Insulation resistance measurements
Insulation resistance measurements pose similar problems because the standard types of test equipment use voltages which are ignition-capable. A similar procedure to that for fault loop impedance or earth resistance measurements can be used.
Intrinsically safe high voltage insulation resistance meters, working at 500 V, are available but the energy stored in a few metres of cable is ignition-capable at this volt- age. Such equipment should only be used strictly in accordance with the manufacturer’s instructions.
The setting of overloads should be checked to verify if they are set to IN. The charac- teristics of the devices should be checked to verify that they are such that they will operate in not more than two hours at 1.2 times the set current and will not operate within two hours at 1.05 times the set current.
Although the example inspection schedules in BS EN 60079-17 include checking flamepath dimensions, it is not normally considered necessary to check spigots, spindles, shafts and screw threads and joints which are not normally capable of being dismantled, unless there is evidence of damage or deterioration.
It is recommended that the settings of overload and excess temperature protection are checked periodically. If experience shows it to be necessary, the tripping characteris- tics should be measured periodically to verify that they are within 20 per cent of the set values.
There is extensive guidance in BS EN 60079-17 for intrinsic safety covering documentation, labelling, modifications, intrinsically safe interfaces, cables, earth continuity and insulation from earth, connections to earth, and separation and segregation of intrinsically safe circuits from each other and from non-intrinsically safe circuits.