The secondary neutral terminal of a 3-wire delta primary and 4-wire delta secondary transformer should not be used for any purpose whatsoever because there is no guarantee that the nuetral current will never exceed 5% of transformer capacity.

The same thing can be said about connecting a generator winding in a closed 4-wire delta configuration. Both generators and 3 legged 3-phase delta delta transformers have very little zero sequence impedance to stop circulating current in a closed delta winding.

Superior transformer configuration for 4-wire delta power are:

1. 2 single phase transformers wired 3-wire T-primary 4-wire T-secondary. This is very easy to build and conventional primary/secondary overcurrent protection can be used. Primary neutral point MUST NOT be connected to source neutral and must be fully insulated from ground. Primary system can be ungrounded.

2. 3 single phase transformers in the 3-wire floating neutral wye primary 4-wire delta secondary configuration. Primary neutral must be insulated from ground and any primary system neutral. Primary system can be ungrounded. A standard single phase transformer is 277 volts primary 120/240 volts secondary. Conventional secondary overload protection does NOT protect the primary windings from overload and does not protect the 2 halves of the center tapped secondary winding. Supplemental overload protection such as motor overload relays arranged to open a primary contactor with conventional circuit breakers providing magnetic tripping for short circuits would work.

3. A 3-wire delta secondary ( or say a 138Y240 volt winding used as a 3-wire secondary ) with a balance coil used to derive a neutral. A balance coil is a single phase version of a zig-zag set for converting 3-wire 3-phase systems to 4-wire wye operation. For a 240 volt secondary a balance coil that is rated 240 volts phase to neutral ( can be made from a standard 240x480 primary 120.240 secondary transformer ) should be used so that ground faults will not cause core saturation. Under normal conditions the 50% applied voltage will give the balance coil some amount of a K rating by running the core cooler.Similar to a zig-zag set a current transformer connected to detect a 50% imbalance in hot lead current should be used to protect the balance coil and the balance coil also needs supplemental overload protection. A secondary circuit breaker that is 4-pole ( protecting the neutral ) would work if the balance coil is big enough.

For gnerators:

4. Connect the generator 138Y240 volts and use a balance coil as in 3 above.

5. Have the generator rewound with a T-connected winding which was a configuration that General Electric one time made. This is essentially an asymmetrical 2-phase generator which produces 4-wire delta power without risk of circulating current in the generator windings.

By the way, 4-wire delta power from a utility is metered as an asymmetrical 2-phase system even though the source and loads are 3-phase.

Mike Cole, mc5w at earthlink dot net

The term "solidly grounded system" when applied to a source of electrical distribution is defined as a system where the neutral is used as a circuit conductor.

A three-phase service feeding only three-phase motors and no phase to neutral loads would not be considered a "solidly grounded system" even though the derated neutral conductor is installed with the service phase conductors .

With the concerns regarding grounding, I continue to be puzzled why some contractors continue to use the Arlington snap in connectors, and inspectors allow them. They do not tighten and the NEC and common sense says that all locknuts, screws, etc. must be tight. The potential for fire, shocks, electrocution, product liability lawsuits by aggressive lawyers is huge. I will welcome any comments for or against these products.

Mike, If I'm not mistaken you have always instructed that the only ground wire on the XO terminal of the seperately derived system (Transformer) is the MBJ. In the picture on this Online Training letter shows the Ground conductor, the MBJ and the equipment grounding conductor. The way I was taught and the way I instruct and inspect is the only conductors on the XO terminal are the Neutral to the panel and the MBJ to a terminal block solidly secured to the case of the Transformer. Please correct me if I am wrong.