Improper neutral-to-case connections in transformers, can cause fire hazards, electrocution, improper operation of protection devices, and power quality problems. Therefore, it’s important to make them only at service equipment and in the transformer only when supplying a secondary panel.
To prevent fire, electric shock, or improper operation of circuit protection devices and other equipment is necessary to stop objectionable current (neutral return current) from flowing on electrical equipment, grounding paths, and bonding paths as required by the Electrical Code.
It is necessary to keep the grounded (neutral) conductor separated from the metal parts of equipment, except as required for service equipment and on transformers.
Making the proper neutral-to-case connections is the key.
Consequences of improper neutral-to-case transformer grounding connections.
There are several consequences of improper neutral-to-case connections that range in severity from problems with equipment to the death of an employee.
If a neutral-to-case bond is made at both the transformer and at the secondary panelboard, neutral current will flow through metal raceways (and on the grounding and bonding path) on its return to the power supply.
When a consumer’s service is supplied by an alternating-current system that is required to be grounded, the system shall be connected to a grounding conductor at the transformer or other source of supply.
The system has to be connected to a grounding conductor at each individual service, with the connection made on the supply side of the service disconnecting means either in the service box or in other service equipment.
It is very important that there is no connection between the grounded circuit conductor (Neutral), on the load side of the service disconnecting means (Service Equipment) and the grounding electrode.
The Electrical Code Rules also permits the use of a system neutral (Transformer Grounding Conductor), derived from a grounding transformer, to ground high voltage systems.
In this case it permits the use of a wye connected transformer to establish a neutral grounded point.
If the secondary of a supply transformer is ungrounded, such as a delta-connected system is directly connected to a grounding transformer, which establishes a neutral grounded system through the grounding transformer.
Grounding of Neutral Grounding Devices (Impedance Grounding).
Where neutral grounding devices are used, warning signs indicating that the system is impedance grounded and the maximum voltage at which the neutral may be operating relative to ground shall be placed at the transformer.
The following are examples of “grounded alternating current systems” commonly used in:
(a) single-phase, 3-wire solidly grounded systems ;
(b) three-phase, 4-wire solidly grounded systems ;
(c) three-phase, 4-wire impedance grounded system.
(a) Single-phase, 3-wire solidly grounded system (midpoint grounded)
In this case the neutral and bonding conductor functions are combined in a single conductor (system grounded conductor) on the line side of the service. But the Neutral (grounded circuit conductor) and bonding conductor function are separate on the load side of the service
(b) Three-phase, 4-wire solidly grounded system (midpoint grounded)
In this case the Neutral and bonding conductor functions are combined in a single conductor (system grounded conductor) on the line side of the service. The Neutral (grounded circuit conductor) and bonding conductor function are separate on the load side of the service.
(c) Three-phase, 4-wire solidly grounded system with no neutral load (3-wire on load side) (midpoint grounded)
The grounded conductor on the load side of the service functions as a bonding conductor with no distributed neutral throughout the system.
The grounded conductor on the line side of the service (system grounded conductor) with no neutral currents is sized as specified for bonding conductor.
(d) Three-phase, 4-wire impedance grounded system (midpoint grounded)
The system connected to ground via sufficiently high impedance and the neutral may or may not be distributed.
(e) Different three-phase, 4-wire solidly grounded systems at a facility (midpoint grounded)
There may be one or more different systems installed at a facility in addition to the system that supplies it (e.g., a generator to supply emergency power, or a transformer to supply a different voltage, to parts of a facility). When these different systems are required to be grounded, each newly established (different) system will be connected to a grounding electrode either at the facility’s service, or at a separate grounding electrode. When separate grounding electrodes are installed, they will be interconnected in accordance with the code.
In addition to the main solidly grounded system that supplies the facility, two different solidly grounded systems are established at the secondary of each stepdown transformer. A neutral of each transformer is bonded to the transformer enclosure and connected to the grounding electrode of the system.
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