Test & Measurement - Restoring safety after the Storm

Typography
Articles

The recent widespread flooding in the UK has undoubtedly damaged electrical equipment worth millions of pounds. In some cases, however, it may be possible to restore this equipment to safe and reliable operation, thereby avoiding the need for costly replacements. Jeff Jowett of Megger looks at what can be done

The key to salvaging flood damaged electrical equipment is to find ways of drying it out effectively, without risking further damage. A number of options are available for this. Probably the most satisfactory is to use a temperature controlled oven with efficient air circulation but, in many cases, this is not possible either because the equipment is too large to be moved to an oven, or because no oven is available.

In these cases, infrared lamps can be used, or a housing can be built around the equipment, with steam coils or electric elements used as the heat source. It is important to make provision for free circulation of air, so that moisture is allowed to escape, and the use of blowers can be helpful.
Another method of heating sometimes used with items like motors and transformers is to pass a current at low voltage through the windings. To avoid the risk of further damage, however, this should not be done until the insulation resistance has been raised to at least 100,000 ohms by other methods. Insulation testers that have kilohm ranges are invaluable in this type of work.

On occasion, welding sets are used as a current source for drying out windings. It’s important to note that these are not intended to supply high currents continuously and they must, therefore, be used at only a fraction of their rated current.

Whichever method of heating is used to dry out the equipment, it’s vital to monitor the insulation resistance for a long enough period to ensure that it has reached a stable value. It is very common, during the drying process, for the insulation resistance to rise to a comparatively high value then dip again. In fact, this rise and fall is often repeated several times as moisture works its way out of the equipment.

While the comments above give general information on salvaging flood-damaged equipment, it is worth looking in more detail at what can be done with various specific types of equipment.

Switchboards and Electrical Controls
- Thoroughly clean and dry out all equipment, dismantling where necessary. After drying, re-varnish all coils. Check contacts for corrosion and oxidation, and make sure that all moving parts operate freely.
- Drain all oil-filled devices, clean them and re-fill with fresh oil of the correct dielectric strength. The oil can be tested for conformance with British Standards using a test set. Dry all insulating barriers, or replace them if they have warped.
- Meters and protection relays will usually have to be reconditioned by the manufacturer. To ensure fast return to service, it may be preferable to fit replacements.
- Clean and dry thoroughly all busbar insulators and control wiring. A minimum of two megohm insulation resistance must be achieved before the equipment is energised, and can readily be confirmed by any good quality insulation tester.
- Check standby batteries for functionality using a battery impedance tester, and check battery straps for corrosion or excessive resistance using a purpose-designed low-resistance ohmmeter.

Electrical Tool and Portable Appliances
- Many of the techniques outlined in the introductory section of this article are suitable for salvaging wet tools and appliances. Before these items are returned to service, however, it is essential that final proof testing is carried out with a portable appliance tester (PAT) in line with IEE code of practice for in-service inspection and testing. As a further precaution, it may also be desirable to flash test Class 2 assets

Rotating Electrical Machines
- Completely dismantle all parts and, except for ball and roller bearings, and either wash them with clean water or steam clean them. Follow this with a thorough cleaning using a grease solvent.
- Thoroughly clean all bearings and housings paying particular attention to oil grooves and reservoirs. Disconnect and swab oil lines, or steam clean them.
- Dismantle the brush rigging and clean the insulators. Some types retain water and must be dried very thoroughly.
- Monitor the insulation resistance of the machine with a modern tester that uses a low applied voltage for the kilohm ranges. Once a value of at least 100,000 ohm is reached, the megohm ranges of the instrument can be used for further monitoring.
- Commutators can be hard to dry out, and it may be necessary to loosen or even remove the clamps to let water out of the inside of the commutator. On large commutators, it may be necessary to use drying temperatures as high as 130°C to achieve effective results.
- Check the bands on armatures or rotors for tightness, as the drying out of the underlying insulation may loosen them. If this happens, they will need to be replaced.
- Some slot wedge materials may be affected by moisture. If this has happened, new wedges must be installed.
- Field coils from DC motors, generators and synchronous machines can present particular problems, and it may be necessary to remove them from the machine for drying in an oven and re-varnishing. After this, the coils should be checked for shorted turns with a digital low-resistance ohmmeter.
- After cleaning and drying, most windings will need re-varnishing. Dip-and-bake varnish is recommended but, it the original varnish is in good condition, an air-drying varnish may be used.
- Before starting the machine, check the entire installation, paying particular attention to lubrication and electrical connections. For three-phase machines, check the phase rotation.

Transformers
- Remove inspection cover plates and check the condition of the windings, looking particularly for signs of failure. Check all connections for looseness and signs of heating. With oil insulated transformers, draw oil samples from top and bottom, and check them with an oil test set. Breakdown should be at least 22kV, or 25kV if an askarel is used. If it is lower, the oil/askarel will need to be replaced.
- Check the insulation resistance. This should be at least one megohm for each 1,000 volts rating, with a minimum of two megohms. Ideally, the resistance should be comparable with the pre-flood values, which may be available from maintenance records. This is best confirmed by an insulation tester with an extended range, such as Megger’s MIT400 and MIT510 products.
- Note the condition of the bushings, external connections, operating switches and protective devices, and take remedial action where needed. If necessary, clean the transformer externally and paint the tank.
- If water has entered the tank, flush the windings with clean insulating oil. If the transformer is small, remove the coil and core, and dry in an oven at up to 90°C. If necessary, dip and bake the windings. Windings for larger transformers can be dried in the tank by forcing hot, dry air (not above 90°C) around the windings after the tank has been drained; by short-circuiting one winding and energising the other with a low voltage; or by using a combination of these methods.
- During the drying process, plot a curve of insulation resistance against time, initially measuring with a low-voltage tester and subsequently, if the process proceeds successfully, changing to a high-voltage insulation tester. If the process is not successful, and the curve shows no sustained increase in insulation resistance, the transformer will need to be re-wound.
- When the insulation resistance has reached an acceptable value, a final test should be made with a transformer turns ratio tester to confirm the transformer has been returned to full performance.

Cables and Wiring
- All open wiring, including non-metallic sheathed cable, can usually be retained after thoroughly cleaning and drying the cable and the junction boxes, and remaking connections.
- Armoured cable will usually have to be replaced, as will lead cable if the ends have been under water.
- Rubber-covered cable in rigid conduit can sometimes be reused, but it must be pulled out of the conduit so that the conduit can be cleaned. The conduit must be thoroughly cleaned to remove all silt and moisture before being used again.
- Check and clean potheads and other insulators, and inspect them for cracks or other damage
- Perform a comprehensive insulation resistance test before returning the installation to service.
Hopefully, this article will have given a useful indication of the measures that can be taken to salvage electrical equipment after it has been subjected to flooding. It is essential, however, to remember, in every case, safety is of paramount importance. This can only be assured by careful testing of the salvaged equipment, during and after the repair process, using appropriate test equipment.

Data Centre news from our sister title

@elecreviewmag