LV Switchgear Maintenance

Introduction

LV Switchgear may fail to protect if not “maintained” periodically during normal running of the plant like guards sleeping on duty if not “checked” regularly. If proper maintenance is not done, the switchgear might appear healthy from outside which, therefore, leads to a delusion that they are normal. But it may lead to an unfortunate accident or a serious fire. So the question comes to mind what is scientific way of carrying out maintenance?

What is Maintenance

Maintenance is a set of organized activities that are carried out to keep the equipment in its best operational condition which will minimize its operational cost and possible future hazards. A common misunderstanding needs to be cleared that 'MAINTENANCE IS NOT A REPAIR'. Repair is the result of neglected maintenance. Maintenance is an art of paying attention to the machinery before it fails & needs repair.

Let's understand difference between planned vs breakdown maintenance (repair).

Unfortunately, the above factors are invariably overlooked. Many users prefer a repair job over a planned maintenance schedule because they believe switchgear are only “carrying” current during normal conditions (a function that makes them appear like switches/cables).

The extent and frequency of scheduled maintenance program will depend on factors like:

• Frequency of operation.
• Application & the Load.
• Type of industry.
• Tripping on fault.
• Relative humidity.
• Presence of dust & reactive gases.

Generally, the gap between maintenance schedules can be anywhere between 6 months and 1 year. It is recommended to do maintenance under following conditions.

• When switchgear has completed specified number of operations or specified period (whichever is earlier). For example, ACB on completing 500 operations on normal load or 1year (Whichever is earlier).
• When switchgear has cleared a major fault.
• If none of the above event take place, all LV switchgear would demand a routine maintenance-cum-test schedule every year.

Did You Know: Switchgear manufacturer generally recommends the schedule for proactive or preventive maintenance. For instance, L&T Electrical & Automation declares activities to be done monthly, quarterly, or annually under maintenance for its Circuit Breakers as mentioned in the product or users' manuals. Adhering to this, enhances product service life.

Objective of Maintenance

The main objectives of maintenance are:

• To ensure equipment reliability & continuity of service, to increase its service period / life expectancy.
• To maximize equipment availability to achieve and deliver the acceptable quality of service & product.
• Increase productivity & profitability.
• Minimize running and operational costs of switchgear devices & equipment.
• To enhance the operator or user safety.
• Increase functional reliability of production facilities.
• To enable desired quality through correctly adjusted, serviced, and operated equipment.

Types of Maintenance

To achieve above listed objectives various types of maintenance are carried out in the field / sites. Following are the main types of electrical maintenance.

Preventive Maintenance

This type of maintenance is performed proactively in advance to prevent failures & to ensure continuity of service; it involves performing routine inspections and servicing. It is categorized by:

1. Time based maintenance is performed at regular time intervals (e.g. number of tripping).
2. Predictive (Condition-based) maintenance is based on the knowledge of the condition of the equipment from routine or continuous monitoring (systematic inspections, measurements).

Predictive maintenance is a technique to help determine the condition of in-service equipment to estimate when maintenance should be performed. Predictive maintenance evaluates the condition of equipment by performing periodic (offline) or continuous (online) equipment condition monitoring.

The goal of the approach is to perform maintenance at a scheduled time when the maintenance activity is most cost-effective and before the performance of the equipment deteriorates.

Predictive maintenance differs from preventive maintenance. It is regarded as condition-based maintenance carried out as suggested by estimations of the degradation state of an item rather than average or expected life statistics, to predict when maintenance will be required.

Did You Know: Temperature module of L&T E&A's Omega ACB continuously monitors temperature of breaker terminal & displays it on release. If temperature reaches beyond the set limit it gives alarm or Trip. This prediction helps in identifying the need for maintenance. L&T APFC relay has feature of counting no. of switching operations & hours of operations. This can help in predicting maintenance / replacement of capacitor duty contactor & capacitors.

Advantage of Predictive Maintenance

Following are the advantages of Predictive maintenance over preventive maintenance.

• Predictive maintenance eliminates unexpected equipment failures.
• It enhances plant safety & minimizes possibility of accidents.
• It helps in inventory & spares management by optimizing the same.
• Preventive maintenance results in cost savings compared to routine or time-based preventive maintenance.
• Time-based maintenance is labour intensive, ineffective in identifying problems that develop between scheduled inspections, and therefore is not cost-effective.
• As predictive inspections are performed while equipment is in service, disruptions in normal operations are minimal.

Corrective Maintenance

It is the maintenance performed after a failure occurs, and it is also categorized by immediate and deferred maintenance. Immediate maintenance is performed immediately as it is being regarded as an emergency; and Deferred maintenance might be scheduled for another period based on the priority of activities.

Good Maintenance Practices

• Room must be clean, well ventilated & dust free.
• Ensure switchboard room to be free from vermin. Cable trenches, bus duct entries are to be sealed to restrict the entry of lizards, rats & snakes.
• Pressurized air circulation system to be used in a contaminated environment.
• To take appropriate measures to avoid leakage & water seepage.
• Entry to the room must be restricted to authorized personnel only.

Maintenance Strategies

An overhaul/maintenance programme, whether on a breaker or any switchgear, would be incomplete unless we recognise the importance of 5 basics of maintenance activities.

• Inspect
• Clean
• Lubricate
• Test
• Tighten

Inspection

It is essential to carry out Inspection in LV Switchgears, which includes:

1. Inspect contact buttons for any signs of erosion, pitting etc.
2. Replace contacts when the thickness of buttons in reduced below the limits specified. The de-ion plates should be replaced at the same time.
3. Inspects the main contacts. Any spots present shall not affect the operation. However, clean the contacts with a fine cloth or CRC-2-26.
4. Check the position of arcing contacts & arc chutes.
5. Ensure ACB ON OFF operation at least 4 to 5 times, also to check proper operation of mechanical parts. Check for healthiness of the protection releases, for example release has self-diagnostic feature, which can be used to understand healthiness of release. In case of error, replace the release.
6. All the components should be examined for cracks, defective plating, frayed wires, burnt insulation, and general overall condition.
7. Look for indications of overheating, arcing or insulation breakdown.

Cleaning

More than half the failures can be avoided if only the equipment is kept clean. Dust and dirt are the worst enemy not only of parts carrying currents, but even those that act as insulators.

Caution: Do not use sandpaper or emery for wiping, as these will unnecessarily remove the contact material. A clean cloth lightly soaked in CRC 2-26 should be adequate.

1. Do not use cotton waste for cleaning.
2. Special attention should be given to insulating surfaces.
3. Compressed air should be avoided to prevent forcing dust and contaminates into other components.
4. High power vacuums should be used to remove all loose debris.
5. Contacts in circuit breakers, contactors, relays and other switching equipment should be inspected & cleaned of all dirt.

Lubrication

The next hazard friction directly results in maloperation, which can lead to failure now or later; and grease is the answer. Before greasing clean the surface thoroughly. Ensure that only recommended grease is used.

Testing

Test switchgear to ensure its proper operation in case of normal / abnormal condition. Testing of switchgear may include the insulation Test & Protection Relay/ Release testing. Insulation Test is recommended using 500V megger for LV switchgear & value of insulation resistance should not be less that 1M ohm. Post IR test, HV testing is performed to check the healthiness of dielectric test.

Tightening

A normal 50 Hz AC frequency & vibrations in the switchboard has a tendency to loosen the power joints, even if they are held by lock washers. Thus, it is equally important to tighten all mechanical joints regularly.

Many times, the terminal looks tightened and it appears as good termination. But a simple mv drop test across the joint could lead to surprising discoveries. Hence mv drop test for termination is the sure means of ensuring better joints because there is more to a good termination than just a tightening the terminals.

Precautions Before Commencing Maintenance Activity

• Ensure only qualified personnel are permitted to use the defeat mechanism & to access an energized compartment.
• Switch OFF incoming power & control supply
• Incoming power devices must be disconnected & de-energized, tagged & locked out.
• Measure the voltage on busbars to ensure that busbars are deenergized.
• Control residual voltage using earthing stick.
• Short all CT secondary terminals.

Important switchgear features to support maintenance

1. Operational counter.
2. Microprocessor Relay / Release with fault history.
3. Temperature module.
4. Switchgears with provision for LOTO.
5. MCBs featuring fault indication.
6. Releases with energy metering.
7. Door Interlocks

Did You Know: Switchgear devices of L&T E&A contain above mentioned features to facilitate maintenance.

Summary

Electrical maintenance is inseparable aspect of switchgear industry which requires educated and qualitative attention at it. Among all the maintenance methodologies, predictive maintenance technique involves monitoring while switchgear is in service. Predictive maintenance strategy is preferred over time based preventive maintenance as it has several advantages like increase in plant safety & prevention of unexpected equipment failures. This will help in improving productivity & profitability. SMART switchgears greatly improve reliability and uptime of plant by providing data required for predictive maintenance.

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L&T Electrical and Automation

A Unit of Schneider Electric India Pvt. Ltd.

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