New approaches to switchgear maintenance have been pioneered in recent years. Simon Garrard outlines which technique is best for a particular application.
New approaches to maintenance of electrical switchgear have been pioneered by the post-privatisation electricity supply industry. These are now finding increasing acceptance in the private sector for low voltage equipment as well as medium-voltage networks.
Assuming that some form of systematic maintenance of switchgear is to be carried out, three approaches should be considered:
- time-based (or scheduled);
- condition-based;
- reliability-centred.
The cost of maintenance is a major factor in these decisions. Too frequent maintenance, based on pre-determined time intervals, can be very costly. On the other hand, if equipment is left until it fails the results could be catastrophic and the cost of unplanned downtime and replacement/repair could far outweigh any savings in maintenance costs.
The effect on business has come to the fore in recent years, especially in modern computer-dominated environments. With many businesses operating 24 hours a day, seven days a week, sometimes at an international level, even a planned maintenance shutdown can prove extremely disruptive and costly.
Modern switchgear should last indefinitely, provided it has been selected to suit the application, installed correctly, and appropriate maintenance procedures are in place. Older switchgear that is in sound working condition does not normally need to be replaced.
However, there are a few situations where consideration should be given to replacing older equipment:
- where switchgear incorporates old rewireable fuses these should be replaced by modern, high rupture capacity HRC fuses or circuit-breakers;
- where equipment incorporates asbestos or polychlorinated biphenyls (PCBs), it should be replaced by specialists and the materials disposed of in accordance with approved procedures;
- circuit-breakers and other devices that show damage as a result of operation under fault conditions – in particular heavy short-circuit currents – should be replaced.
The personnel touch
Only trained authorised personnel should be allowed to carry out maintenance work. This may be a company’s own facilities managers, or an electrical contractor. However, in many cases facilities management staff may not have the necessary skills or time to pursue a systematic maintenance policy. It is here that the skills and experience of a specialist maintenance company may be an advantage.
The maintenance company should have the experience to determine the most appropriate regime for the equipment in question. It should be capable of maintaining a wide range of equipment from different manufacturers and will have an accumulated knowledge of operational characteristics for each product under differing operating conditions.
Reliability-centred maintenance
Reliability-centred maintenance relies on statistical sampling of equipment. Typically around 10% of units are subjected to detailed testing according to a time-based programme. The results are used to determine if and when the other units should be serviced.
Where should it be used?
This approach suits situations where there is a high population of identical products. In utility applications it has been used for oil-filled mv switchgear, but it is also suitable for lv equipment that is not liable to deterioration due to frequent or onerous duty.
What are the procedures?
For oil-filled mv switchgear, statistical sampling may be carried out every six years; however more frequent attention (annual) is advisable for lv equipment such as switches and disconnectors, circuit-breakers etc. The samples should be representative of the entire batch in terms of frequency of use, operating current etc. Different samples should be chosen on each occasion. Careful record-keeping and a procedure for interpreting the results into a maintenance schedule for the whole batch are necessary.
One regional electricity company points out that this has eliminated oil changing on 90% of its stock of oil-filled switchgear, with big savings in costs. However, periodic visual assessment has been stepped up.
Time-based maintenance
Time-based maintenance is the traditional approach involving maintenance at regular scheduled intervals.
Where should it be used?
It is particularly appropriate to lv distribution systems involving fused switches, moulded-case circuit-breakers, miniature circuit-breakers etc. Some devices may remain unused for many years because they are there for emergency use, but when they are called upon to operate, correct functioning will be crucial. Contacts that have been unused for a long time may become dirty or undergo chemical changes that could ultimately lead to arcing.
What are the procedures?
The fundamental requirements are:
- operate all switches and circuit-breakers annually;
- lubricate switching mechanisms annually;
- check all terminals annually;
- operate all rcds quarterly.
Annual operation of switches and circuit-breakers confirms that they are still functioning, exercises the mechanism and cleans and lubricates the contacts. Moulded-case circuit-breakers should be tested by switching off and on manually several times. The trip-to-test facility should be tried with the main switch on. Shunt trips, undervoltage releases, auxiliary/alarm contacts and command devices such as time switches and contactors should all be tested.
Annual maintenance should also include a visual check. Any scratches on equipment enclosures should be touched up to prevent corrosion. Seals and gaskets should be checked, especially when closing enclosure doors. Any build-up of dust should be removed.
Condition-based maintenance
Condition-based maintenance relies on monitoring parameters that indicate the condition of the equipment and using this to determine the appropriate time to carry out maintenance. The use of non-destructive testing to predict plant failure is well known, but it is rarely used to assist in determining plant maintenance periods. However, this has changed with improvements in the reliability of non-destructive apparatus and the accumulation of data over the years.
Where should it be used?
Condition-based maintenance is appropriate to mv equipment such as oil-filled switchgear where annual changing of the oil is unnecessary, but dissolved gases or chemicals in the oil could have disastrous effects if left undetected. It is also appropriate to mission-critical low-voltage switchgear, where it enables incipient faults to be detected so that remedial action can be scheduled for times when disruption to business will be minimal.
What are the procedures?
Oil sampling is well established for testing for evidence of corrosion or wear in oil-filled equipment. A small sample is drawn off for analysis rather than changing the oil completely.
Infrared detection, or thermographic measuring, is another non-destructive procedure used to identify ‘hot spots’ caused by terminations that have worked loose or components that are overheating. The equipment is scanned by the means of a handheld thermographic camera. Some technical skill is required to interpret the results correctly – for example an undervoltage coil that is operating correctly will generate some heat and show up as a ‘hot spot’ in thermographic charts.
Partial discharge testing is a relatively recent technique that can be used to predict possible switchgear and bushing failures by sensing partial discharges in faulty equipment. This involves the permanent installation of sensing equipment within the plant so is appropriate to expensive or mission-critical equipment.
Condition-based maintenance has enabled some operators to reduce the cost of switchgear maintenance by 80%, from about £650 per unit to £110 per unit, by extending the intervals between servicing of oil-filled 11 kV switchgear.
Source
Electrical and Mechanical Contractor
Postscript
Simon Garrard is service manager with Eaton Electrical Services & Systems.
