Restoring water-damaged electrical equipment

01/31/2006


Editor’s note: look in the February issue of PLANT ENGINEERING magazine for more information on disaster recovery as well as exclusive coverage on how one company bounced back from the devastation of Hurricane Katrina.

In the popular mind, water and electricity are considered incompatible, and for good reason. With the presence of dissolved ions, water provides an excellent conductor — but not generally where current is intended to go. Water can short circuit, break down and ultimately destroy electrical equipment. However, this threat should not be taken as absolute.

Electrical equipment is frequently recoverable from water damage — even to the extent of flooding and submersion. Do not automatically assume that such equipment must be replaced. Water can be gently driven off, and equipment restored to operable condition.

The operative term here is “gently.” Temperature must be carefully regulated in drying operations. Fairly high currents may be used to provide “warmth,” but at least initially, at only a few volts. Higher temperatures that may convert water to steam must be avoided, as this can puncture insulating materials and require expensive repairs.

Effective drying techniques include:

  • Oven with suitable temperature control and air circulation

  • Infrared lamps

  • Electric heaters within a suitable enclosure, with openings for circulation

  • Vacuum drying, but this requires specially trained personnel.

  • Circulation of low-voltage current through windings — must not be done until insulation resistance has reached at least 100,000 Ohms. As equipment dries, insulation values typically rise. Welding sets can provide current, which should be limited to a fraction of nameplate rating, and a careful check kept on temperature. High-voltage ranges on insulation testers should not be used to make these initial determinations of insulation condition. Localized heating could damage the insulation. Quality testers will also have a low-voltage kilohm range. All drying operations should begin with measurements made on this range, with high-voltage tests performed only after resistance has risen to several megohms.

    • Do not discontinue drying as soon as a high resistance is attained. Moisture dispersal throughout equipment may be erratic, and localized drying can cause sudden leaps in measured values. Continue drying until acceptable values are steadily maintained.

      The following procedure is typical for rotating equipment; it can be appropriately modified to fit other electrical restorations:

      1. Dismantle all parts.

      2. Wash with clean water or steam clean (except bearings); follow with a suitable grease solvent.

      3. Bearings and housings are washed or steam cleaned, but be careful not to remove necessary lubrication.

      4. Dismantle brush rigging and clean insulators.

      5. Dry equipment as described above, monitoring effectiveness by insulation measurements, starting at low voltage (~5 V). An acceptable rule for insulation values is 1 megohm of resistance (IEEE has recently increased this recommendation to 2 megohms) per 1000 V of rated operating voltage, and never less than 1 (2 by revised standard). However, this is only regarding basic operation. It is a good idea to achieve much higher values for prolonged service.

      6. Commutators can be difficult to dry; it may be necessary to loosen or remove clamping nuts to let water drain.

      7. Check bands for tightness and slot wedges for moisture.

      8. Some coils may prove difficult to restore. After cleaning and drying, they may be immersed in insulating varnish while hot and allowed to cool while immersed.

      9. Once dried, dust can be removed with dry compressed air, at &40 psi.

        1. Some special points for cables and wiring:

          1. Open wiring can generally be retained after effective drying — including junction boxes — and reconnection

          2. Armored cable and lead cable where ends have been submerged will usually have to be replaced because their construction hampers effective egress of moisture.

          3. Rubber-covered cable in conduit can generally be restored, but must first be removed and the conduit thoroughly cleaned and dried.

            1. Remember that some equipment (e.g., transformers) may be protected by insulating oil. This can become contaminated by water and dirt just as dry insulation. Appropriate cleaning or replacement procedures should be employed, followed by a proof test. Insulating oil is tested for electrical breakdown, generally at values around 22 kV, with some variation based on type.

               



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