With intrinsically safe circuits, maintenance activities are performed with the power (usually 28 V maximum voltage) on. However, when there are circuits protected by barriers, those barriers could be permanently short-circuited. This could cause a fuse to blow, rendering the barrier useless. So, always ensure that the barriers are not short-circuited and all apparatus are safely connected and have no improper contacts. When possible, remove the apparatus (particularly easy with plug-in type connectors) and check them in a lab. When it is impossible to remove the apparatus, disconnect input/output conductors and, after identifying them, connect them temporarily to the isopotential ground, or, if already grounded, keep them isolated and anchored to the free terminals of a supporting terminal block. Then connect a calibrator to the input and a calibration indicator to the output. Both must be certified for the division and gas group in which they are used. After completing verifications and calibrations, restore input and output conductors with extreme care.
Apparatus failure and repair
Decreasing downtime for repairs is key to reducing maintenance costs (see CP, October, p. 25). Smart planning during design — such as selecting modular and plug-in type field and control-room instrumentation and ensuring adequate spare parts to permit immediate substitution — will help to reduce your downtime and make repairs easier.
Repairing explosion-proof apparatus. When operating problems or machine failure involve explosion-proof plants, it’s vital to determine as soon as possible the cause of the problem, identify and isolate the problem area and substitute or repair malfunctioning components. Always keep in mind that only authorized personnel must be permitted to repair explosion-proof apparatus, because equipment must not be serviced under power.
Even though diagnosing explosion-proof plants involves the same procedures used elsewhere, there is always danger of fire or explosion. Therefore, keep these points in mind:
• Do not perform connections not shown in the plant’s schematic, unless the risks relative to safety have been analyzed.
• Only use test instruments certified for the same hazardous location and gas group as the circuits to be analyzed.
• Isolate the part of the plant where repairs must be performed. Consider the effect of the tests on interconnected circuits.
• Most importantly, do not cross or eliminate the safety protections that are present in the safety barriers and in other parts of the plant.
Repairing intrinsically safe apparatus. Working on intrinsically safe apparatus does not have to compromise the characteristics of intrinsic safety. The most frequent ways to ensure that apparatus are intrinsically safe are:
1. surface distances between the main line and the intrinsically safe circuit, and between two different types of intrinsically safe circuits;
2. protective coatings that increase the insulations to more than those obtainable with distances only;
3. protective fuses on main and signal transformers, and output circuit barriers;
4. signal and main transformers with dielectric rigidity that has been individually tested, and with distances and insulating or isolating materials that are guaranteed;
5. barrier resistors with construction techniques, nominal powers, values and tolerance as per the certification documentation;
6. diode or zener barriers with nominal voltage, tolerance, nominal power and assembly polarity well-defined;
7. optoelectronic coupler that is certified as a component having surface and internal distances and approved construction techniques;
8. electromagnetic relays that are certified with guaranteed surface distances between the coil circuits and the contacts or terminals for armor ground connections; and
9. functional modules, encapsulated or not, that are certified as components that are compatible with the concept of intrinsic safety.
There are some additional points to remember if an intrinsically safe apparatus must be repaired in an emergency situation. Do not modify air and surface distances of the barriers and their components. Do not substitute any component that determines its intrinsic safety (usually marked with shading on the silk screen of the printed circuit boards and on the schematics). Only replace fuses with identical ones (rapid, medium lag, etc.) and nominal current. Do not substitute main or signal transformers, or certified modules unless with identical components supplied by the manufacturer of the apparatus. Take care in reciprocal positioning of the components and in repositioning eventual isolators or spacing collars placed on component terminals to distance them from the printed circuit board. Carefully verify the repaired card or component, to ensure the complete efficiency of all intrinsically safe protection. If soldered, make sure to accurately clean the printed circuit board and restore protective coatings.
It is often difficult for every manufacturer to supply all of the documentation necessary to perform repairs on intrinsically safe apparatus. The safer and sometimes less costly solution is to keep a series of spare cards or modules that permit an immediate substitution of the faulty unit. The defective unit can then be sent to the authorized service dealer where it can be repaired.
Rounding out the program
In addition to the previous maintenance criteria, always adhere to established safety practices — e.g., tell the control room operators when you are working in an area and follow lockout procedures without fail. Safety meetings, held on a regular basis, offer an opportunity to discuss various scenarios and ensure that all personnel are familiar with the correct procedures for each specific hazardous area within your plant.
Implementing effective preventive measures and training staff and operators to work safely while developing the capability to handle emergency situations is the key to continuous safe operations.
Joe Kaulfersch is a market analyst for Pepperl+Fuchs, Twinsburg, Ohio. E-mail him at email@example.com.