Delta-Wye Conversions

What should happen when the electrical utility or a utility customer decides to convert the ungrounded 600 volt, 3-wire supply to a 600/347 volt, 4-wire, solidly grounded electrical supply? Some commercial and industrial businesses still prefer to use an ungrounded 600 volt supply for service continuity reasons, or to avoid the costs of converting to a grounded 4-wire supply. Sometimes voltage conversions are carried out when one or more businesses in a multi-unit complex want to install 347 volt lighting or other single-phase loads, or when the electrical utility decides to upgrade its distribution system for other reasons. Unless notified, some utility customers may be completely unaware of the changes and the resulting consequences. Others may decide to convert without sufficient analysis, and without taking the correct steps to avoid problems. Without careful planning, unexpected problems can occur, including electrical equipment damage, injuries or worse.

Such problems are easily avoided with some special precautions.   Switching over to a solidly grounded supply normally has advantages. The most obvious, a grounded electrical system is absolutely necessary where single-phase loads must be supplied. A solidly grounded system provides the best control over phase-to-phase and phase-to-ground voltages, preserving the life of electrical equipment and wiring by minimizing exposure to overvoltages and capacitive arcing faults. This usually more than offsets the inconvenience of an odd unplanned power interruption, since these can usually be minimized by a regular preventive maintenance program.

Another thing to keep in mind, the available fault level is usually higher after the conversion. This means that the existing overcurrent protection may have become obsolete. The most sought after benefit of ungrounded 600 volt systems, they don’t trip on a single-phase ground fault. Everything continues to run with a ground fault on one phase. However, a second fault on a different phase can cause massive equipment damage, injuries or worse.

The Canadian Electrical Code requires that ungrounded systems must be equipped with ground indicating lights [Rule 10-106(2)] to warn of the presence of a ground fault, so that repairs can be done as quickly as possible. However if warning lights are frequently ignored or neglected, extended overvoltages combined with capacitive arcing faults will cause insulation damage and premature electrical equipment failures. A number of electrical code requirements and safety precautions need to be observed when the electrical utility or the user converts from an ungrounded to a grounded electrical system. When 4-wire loads are to be supplied:

• The electrical utility’s 3-wire metering is replaced by 4-wire metering.

• Circuit-breaker and fuse protection throughout the facility must be carefully reviewed and, when necessary, upgraded to safely interrupt the available fault level, that will often be higher after the conversion [Rule 14-012(a)]. Combined with a higher possibility of ground faults, this could lead to more frequent damage and longer shutdowns if neglected.

• The electrical system should be checked over for weakened insulation and existing short-circuits before reconnection to the grounded supply.

• Remove the ground indicating lights. They could deceive someone into believing the system is ungrounded and making some wrong decisions.

• If the 600/347 volt electrical supply is 1000 amperes or more, ground-fault protection will be required by the electrical code (Rule 14-102). This could mean replacing fused main service switchgear with a circuit-breaker type or adding ground- fault protection to an existing main service circuit-breaker.

• To be compatible with the new electrical supply and in compliance with the electrical code, the main service equipment must be equipped with a neutral block to accept the electrical utility’s fourth wire, and for system grounding purposes. This also applies throughout the facility at points where single-phase loads are connected (Rule 4-026).

When the electrical utility changes the 600 volt supply to 4-wire, and there is no need for single-phase loads, at least the following precautions are necessary:

• Depending on the electrical utility’s practices and policies, metering may be converted to the 4-wire type or continue as 3-wire metering.

• Overcurrent protection through-out the facility should be reviewed and upgraded if necessary, since the electrical system fault level is usually increased.

• Any ground indicating warning lights should be removed to prevent confusion.

• In some jurisdictions the electrical utility’s neutral conductor may be connected to the service equipment enclosure. Otherwise, a neutral block in the main service equipment may have to be provided.

• Also, check whether the electrical inspection authority will require ground-fault protection should the 600 volt main service equipment have a rating of 1000 amperes or higher.

As with previous articles, you should check with the local electrical inspection authority for the most accurate interpretation of any of the above in each province or territory as applicable.

Leslie Stoch
Leslie Stoch, P. Eng, is principal of L. Stoch & Associates, providing electrical engineering and ISO 9000 quality systems consulting. Prior to that, he spent over 20 years with Ontario Hydro as an electrical inspection manager and engineer. Les holds a B. S. in electrical engineering from Concordia University in Montreal.