3 Phase 3 Wire 480 Volt Ungrounded System

[wirestu]

We have been asked to install a 250 kW emergency generator set on an existing 200 Amp service. The service is a 3-phase 3-wire 480V 200 Amp service from three customer owned individual 37.5 kVA step-down transformer pots (13.2kV-480V). The transformer primaries and secondaries are wired in delta-delta. The frame of the transformer pots are bonded to the metal gate and fencing enclosing the transformers, the metal conduit enclosing the service entrance conductors and connected to a driven rod. There is no apparent connection from the transformer windings to the conduit/ground rod.

There are two delta-wye step down transformers inside the building.

My questions are as follows:

1) When we took voltage measurements with a high impedance digital multimeter, we obtained the following readings: Phase-to-Phase on all three legs = approximately 480 Volts evenly balanced. When we took phase to the metal conduit voltage readings the values were 205, 295 and 260 Volts. How are we able to obtain voltage to ground readings even though the conduit/ground system is isolated from the transformer windings? If this is a phantom voltage, what method could be used to verify that the system is indeed isolated?

2) According to the NEC what are my responsibilities as the installing contractor with regard to the existing service? I know a new service at the potential needs to be grounded, but I am not directly touching the existing service.

3) What are the safety concerns if the existing system is left as is?

Thanks for all replies.

Stu

 

[bennie]

Re: 3 Phase 3 Wire 480 Volt Ungrounded System

wirestu: You are describing a true and only separately derived system. The premises wiring is not solidly connected to the utility source, magnetic coupling is the only method of energy transfer.

An engineer can design a zig-zag grounding system, if determined to be necessary, other wise leave it ungrounded. A corner ground should not be done.

The readings are normal, the result of capacitance coupling.

This appears to be a specialized power system. What is it supplying?

 

[dereckbc]

Re: 3 Phase 3 Wire 480 Volt Ungrounded System

Wirestu

1. The voltage readings are normal. As Bennie stated the voltage you are seeing is from capacitive coupling.

2. The facility needs to be grounded per NEC 250 for ungrounded systems if it is going to be left ungrounded. Basically the same as a grounded system. You will need to establish a ground electrode, and distribute with phase conductors.

3. In many respects the ungrounded delta is safer than a grounded system. However these systems are used where AC service is critical and where outages due to phase to ground faults are minimized. These type of systems are installed where only qualified personnel will service the system. They require ground fault detection systems. As Bennie stated only a PE shopuld be designing the system. You as a contractor should only be responisible for building the design, but not designing the system.

 

[wirestu]

Re: 3 Phase 3 Wire 480 Volt Ungrounded System

Bernie,

The facility is a water pump station on an old Navy site that they have turned over to the public. The primary power comes from a Navy owned steam plant which drives high voltage generators. The 480 V secondary service drives a well pump motor. I don't know the details of the source of power. There might be ground fault detection on the primary feeders, but I would not think this would satisfy the ground fault detection for the service?

Derek,

There is no ground fault detection on the secondary service. The site is limited to authorized personnel only.

Is the rigid metal conduit enclosing the service conductors acceptable as the service ground? It is tied to a ground rod and all other metal parts of the transformer and fencing. If it is not would I merely install a new electrode and conductor? What is the purpose of this as no fault current will be returned to the source under a ground fault?

Thanks again,

Stu

 

[bennie]

Re: 3 Phase 3 Wire 480 Volt Ungrounded System

Wirestu: You have hit me where I wanted to be hit for a long time. (I'm not a masochist) This is an example of technology that has become corrupted by time.

An ungrounded secondary does not have a fault path. An ungrounded service does not have a ground electrode conductor. The metal enclosure should be connected to earth. The tank of the transformer should be connected to the earth. The tank and service enclosure should not be bonded.

An ungrounded power supply is safe. An LG fault on one phase, creates 480 to ground on the other phases. This is the major hazard. A second fault becomes more likely with the stress. A second LG fault becomes a LL fault.

The reason for not bonding between the transformer tank and the service equipment is a LG fault at one end will not appear as an elevated voltage on the other end.

The grounding of the wye secondary should follow the procedure for a separately derived system. This is no different than a service ground electrode system.