Today's manufacturing plant requires a lot of expensive maintenance to maintain optimum output. James Hunt examines factory maintenance from an electrical contractor’s perspective:
Modern factories are complicated things. Faults will occur in the real world and they need to be diagnosed and repaired quickly. So, accepting that good and regular maintenance is key to the successful running of any factory, what needs to be examined from the point of view of the plant electrician or electrical contractor?
Criteria include whether the site electrical staff carry out the work or whether it is sub-contracted out, and the nature of any contract itself. For example, is it a 'four-walls' contract? Does it cover all electrical equipment - such as the production equipment itself - or only the plant electrical systems? Assuming that it is the latter, the equipment to be maintained may include:
- AC / DC machines.
- Cable terminations.
- Circuit fault finding.
- Control equipment / drives.
- Electrical safety and test instruments.
- Fire & security systems.
- Hazardous area equipment.
- Instrumentation, alarms and data-logging.
- Lighting and LV equipment.
- Switchboards, switchgear and protection.
Factory maintenance work may cover preventative maintenance to plant / machinery, general repairs / renewals and testing / recording and routine maintenance. It is essential that the maintenance engineer or contractor is experienced in basic AC/DC theory, schematics & electrical print reading, control equipment, power supplies, construction, installation & distribution, computers & PLCs, mechanical equipment, hand & power tools and, of course, test instruments. It is also useful to have some knowledge of digital/analogue electronic systems.
A periodic maintenance schedule will help obtain best equipment service. Annual checks must be made on all major devices following installation. Once you have established any trends, the maintenance intervals can be extended to as much as 36 months but inspections should be frequent. Permanent maintenance records must be kept and schedules adhered to. Keep a library of equipment manufacturers' instruction books.
All recognised safety practises should be followed and all work should be carried out by properly qualified electricians or contractors. It should be thoroughly inspected and tested to the I.E.E. regulations. Upon completion of the work, all relevant and completed NICEIC forms must be passed to the client.
Predictive maintenance:
A useful way of predicting equipment failure is to look for abnormal hot spots. Increases in vibration or noise or a change in frequency are also warning signs. A hot spot could be caused by many things but examples include poor busbar connections or worn motor bearings. Thermography, using infrared (IR) imaging cameras, detects potentially dangerous hot spots. Almost everything gets hot before it fails, making IR cameras very cost-effective and valuable diagnostic instruments. Vibration monitors are also useful for predicting failure.
Maintaining systems:
1) Back-up power & standby systems - Routine maintenance (sometimes daily) is needed. Records must be kept. Check all battery systems. Transfer switches - clean, tighten connections, inspect and test for overheating and contact erosion. Monthly full-load test for 30 minutes and back-up power to be tested at full critical emergency load. Check generator/engine cold start. Signalling systems - check and clean occasionally. Fire, smoke, IR & other detectors - check periodically, clean & test for functionality. Skilled personnel may be needed.
2) Busbars - Check especially for loose connections (look for discolouration). These cause failure through local overheating; do a thermographic survey. All connections should be torqued correctly. If there is local vibration, check more often. Arcing means that the connection must be broken, cleaned/repaired, then re-made. Don’t clean plated aluminium busbars with abrasive. Clean, then check carriers and insulators for cracking and arc tracking.
3) Cable ducts / trunking, busways, underfloor systems - Check for oil and water or other factory fluids - clean. Note - insects and rodents attack insulation! Clean ventilated busways using compressed air. Ensure that ventilation is acceptable. Check all plug-in devices. Don’t re-torque connections unless they are loose.
4) Earthing - Periodic checks for low resistance to earth for all systems. Also check for loop flow. All connections must be clean and tight.
5) Enclosures and switchgear - Power off and properly ground busbar. Vacuum clean. Check all fasteners for tightness - use torque wrench where necessary (especially busbars and breakers). Lubricate using non-hardening, non-conductive grease. Inspect safety shutters, limit switches and interlocks. Clean and inspect environmental systems. Inspect all internal equipment such as switchgear, fuses, contactors and relays for correct operation. If fuse continuity is greater than zero, it is blown. Check all fuse contacts and also new fuse continuity. Check all wiring connections. On primary disconnection devices, look for wear. Remove sulphide deposits on silvered surfaces using alcohol or silver cleaner. Afterwards, measure resistance to earth and between phases. Watch especially for any change in resistance over time. Apply contact lubricants to stationary studs and breaker primary disconnects. Ensure that blast shields are correctly fitted.
6) Lighting systems - Today, lighting resources now need to be managed to maintain lighting quality. Illuminance will need to be measured from time to time. Lamp life will often be reduced though system over-voltage while under-voltage may reduce light output - check the line voltage. Clean fittings and lamps regularly; they can get dirty quickly in many factory environments. Carefully dust luminaire aluminium parabolic louvres, then clean using Isopropyl alcohol or an aluminium reflector cleaner. Use gloves - fingerprints can etch surfaces! Water wash polystyrene or acrylic louvres. Planned group re-lamping can save money, improve efficiency and cut failed equipment search times. Ballasts last a long time as long as they don’t get too hot. Their failure rate is small over the first 70% of average life but will increase beyond that, so maintain a log of installation dates. Predicting failure saves money. Lighting wiring and busbars should be maintained as for other distribution systems.
7) Motors - Generally, follow manufacturers' instructions. Both AC and DC motors benefit from periodic inspection plus preventative maintenance services. Before invasive maintenance, ensure that motor is de-energised, tagged and locked. Clean. Checks will include lubrication, brush problems, hot spots and loose belts. Investigate unusual vibration/noises. Larger motors can be repaired regarding replacing / repairing bearings, commutators, collector rings and brushes but rewinds are specialist jobs. After testing, discharge before handling test leads.
8) Motor controls - electromechanical - De-energise and clean, check as scheduled or as required for: unauthorised attachments; defective outlets/switches; correct fusing/blown fuses; panel overheating; lamps and indicators; correct ground resistance; under-voltages and PF. Check operation of all related breakers. Check thermal overload relays periodically. Do a dummy load test. Contactors should also be examined and copper contacts can be cleaned up using a fine file. Don’t file silver contacts. Ensure that contact pressure is correct and that all connections are tight. Coils should not show signs of overheating; magnets should meet specifications. For VS drives, experienced personnel may be needed.
9) Power cabling - Check for: sharp bends/excessive tension; physical damage; water submersion; damaged fire-proofing; insulation swelling/soft spots; poor earth connections/high impedance to earth and mutual overheating. Re-route if necessary. Overheating can be caused by low PF and un-balanced currents - check. Check all connections and fittings. For LV cables, de-energise, disconnect at both ends; then carry out cable insulation resistance tests or, for MV/HV cables, DC over-potential tests (only trained personnel !). A cable fault locator can find a specific fault's position.
10) Power circuit breakers (all except moulded case and breakers for control) - Gain access to and de-energise. Test for correct operation of mechanisms and for overheating. Insulation must be clean and dry. Clean inter-phase barriers - use solvents if necessary. All contacts should be cleaned carefully, checked and adjusted. Arcing bumps can be removed by careful filing. Remove deposits using alcohol or silver cleaner. Don’t lubricate arcing contacts! Check all mechanisms for correct operation and excessive wear. Disassembled moving parts should be lubricated with non-conductive grease.
11) Transformers (dry-type, small power) - Transformers failure is rare but serious – so carry out regular maintenance. Inspect for dirt annually, especially on windings and where it impedes airflow. Testing can be carried out in service. All measurements should be taken at peak load and recorded - to include load current, voltage and ambient temperature readings. When transformer is not in service, warm to remove moisture. Check insulation for tracking/carbonisation and for loose insulators and connection corrosion. Connections must be tight.
If the above is carried out regularly and carefully, there should be little likely-hood of unexpected equipment failure, bringing peace of mind to plant manager and maintenance personnel alike.
Test instruments and applications:
- Feeler gauges - Check generator air gaps.
- Clamp meters, DMMs, ohmeters etc - Circuit voltage, resistance & power.
- Current transformers (CTs) - Reading HV & high current circuits.
- Insulation resistance meters -Test / monitor insulation resistance.
- IR imaging camera - To detect hot spots.
- Low resistance tester - Low resistance paths for contacts.
- Light / illuminance meter - Measures light intensity.
- Motor rotation tester - Checks for motor rotation direction.
- Oscilloscope - For testing electronics.
- Phase tester - Checks circuit phase rotation.
- Portable capacitance - Accurate measurement of capacitance.
- Resistance bridge - Accurate measurement of resistance.
- Recording meters (various) - Monitors and records parameters.
- Specific gravity (SG) tester - Checks battery SG.
- Spring tension scale - For checking brush/contact pressures.
- Tachometer - Checks speed of rotating machinery.
- Thermometer - Temperature correction.
- Variable autotransformer - Reduced voltage circuit tests.
Some specific test instruments: ADWEL’s Model WRT-100 winding resistance test set quickly and accurately measures highly inductive transformer, motor and generator resistance.
FLIR supplies its ThermaCAM IR cameras and Reporter software which enable quick, non-contact location of hot spots. Their analysis capabilities also quickly enable an understanding what action to take, and when.
Fluke’s 179 easy-to-use true-rms DMM is an ideal general-purpose maintenance tool. It also comes with a thermocouple temperature probe ( -40ºC to +260ºC). The company’s 190B series ScopeMeter high-performance hand-held monochrome oscilloscopes are ideal for troubleshooting too. A ‘Stop-on-Trigger’ function allows recording and analysing power interruptions, machine failures and UPS-take-over.
Megger’s Series MEG10-01 portable 10kV insulation test set meets IEEE43:2000 ‘IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machines’, which permits 10kV insulation testing for 750W motors or larger. Also from Megger is its BITE 2P battery impedance tester which greatly reduces the need for time-consuming discharge tests and eliminates battery stress.
Where factory lighting systems need to be checked for illuminance, Minolta Industrial Instruments offers its portable handheld T-10 illuminance meter.
Robin’s KTS 1620 kit, comprising portable tester, holdall and accessories, provides the five primary test functions required by BS 7671:2001. This digital multifunction tester carries out continuity, insulation, loop impedance, RCD and prospective short circuit (PSC) testing.
Wavetek Meterman Test Tools’ true-RMS 38XR DMM is ideal for industrial automation and control troubleshooting. It offers optional real-time datalogging by PC and measures volts, amps and ohms, temperature, capacitance, frequency and 4 - 20mA loop current %.
This article was published in Electrical Times magazine and has been used courtesy of publisher and Voltimum UK partner Highbury Business Communications.
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