Corner grounded delta service enclosure bonding/grounding requirements

[BlueLineElect]

Hello,

I have a 50 year old electric utility sourced 3 wire 3 phase corner leg grounded 240V delta service (100A) with no grounding/bonding of the grounded phase (B) done in the service enclosure.

Should I provide main bonding jumper (250.28) and grounding electrode conductor (250.24(A)) connections to the grounded delta phase conductor. Further, a 3 pole service disconnect (breaker) is disconnecting all service conductors. I'm considering removing the grounded phase conductor from the service disconnect breaker and installing a neutral style bus to route the grounded phase conductor through (line and load connections), where the MBJ and GEC would also connect.

Please advise as to if my proposed changes would meet NEC.

 

[electrofelon]

A corner grounded system would like pretty much like a single phase service and be as you propose: only two of the phases run through the circuit breaker, and the grounded phase landed on a bus with any other grounded conductors, the grounding electrode conductor, and the main bonding jumper. It really throws the newbies off when they start making voltage measurements on these

 

[ptonsparky]

That grounding/bonding can be done at the point of attachment, or at the meter.

 

[electrofelon]

That grounding/bonding can be done at the point of attachment, or at the meter.

Just to clarify, the GEC can land anywhere from the service point to the service disconnect and need an MBJ at the service disconnect.

 

[Elect117]

Installations and Inspections of Corner-grounded Systems

iaeimagazine.org

This might help.

If you plan on grounding one of the phases, you might want to consult with the utility. It can sometimes be difficult to figure out which one they have grounded. On old ungrounded systems, you never know if it is the correct corner or a ground faulted conductor.

Since you are from southern California, I would also add that there were times when 240V 3W 3ph and 120/240 1ph Services were ran to the same building and feed off the same utility transformer. This can create a improper grounding scheme on the utility's connections. Where the center tap was grounded but the corners weren't because it would create a 120V ground fault between the corner and one of the single phase conductors. Or the grounded conductor was the "208V leg" and the center tap was also grounded. I have seen both. Most likely they are two different utility transformers, and no faults, so you can simply find the corner the utility has grounded.

 

[ptonsparky]

Just to clarify, the GEC can land anywhere from the service point to the service disconnect and need an MBJ at the service disconnect.

Glad someone spits it out properly.

 

[BlueLineElect]

A big thank you to all the insightful responses, they really educated me.

I have rechecked that phase B is effectively at ground potential (at the utility as there's no bonding of phase B on premises - thus my posting motivation). It is a center grounded 3 phase (240V) system. Phase to ground voltages are either 240V (Phase A & C) or 0V (on Phase B only), and 240V phase to phase (on all).

@Elect117 that's an interesting point about a utility supplying two separate services (a 3W 3P) and (3W 1P) from the same (center tapped delta) transformer. Resulting in there not being provided the center-tapped grounded & 'neutral' (4th service conductor) at a 3W 3P service, and such could lead someone (whom wasn't paying attention) into problems if they assumed either all conductors have no reference to ground (i.e. an ungrounded system), or they had a corner grounded. But fully checking all phase-phase and phase to ground voltages will establish the true situation (the 208V and 120V to ground in phase-ground tests would demonstrate that the 3W 3P service is coming from a center tapped system).

@Elect117, thanks! this link, https://iaeimagazine.org/2002/2002january/installations-and-inspections-of-corner-grounded-systems/ was very informative (I obtained the latest Soares Grounding & Bonding Series a few days ago, after the OP, and see that the linked article, by IAEI whom publishes Soares, draws from Soares on this material - for those wanting to go deeper).

The (50 year+ old) installation that prompted my OP, has no bonding of the grounded conductor at the premises (and it's routed through the 3 pole main service breaker, Arrow-Hart/Murray, haven't checked yet for split voltage rating of it, but doubt that's an issue).

I now understand this installation especially needs the MBJ to address the concern (adequate ground-fault clearing), from IAEI article link above, expressed here:

The grounded conductor connection in the service disconnect serves two vital purposes. In normal operation, the grounded conductor, which is usually the neutral conductor of the system or service, will carry the unbalanced return current to the source. In ground-fault conditions, the grounded conductor is required to serve as the low impedance path for fault current back to the serving transformer. The serving utility provides service that includes a grounded conductor, but very rarely an equipment grounding conductor. Sizing this grounded conductor must include provisions to carry the anticipated load current and any fault current for clearing overcurrent devices. The path for fault current must be effective, permanent and continuous, have ample capacity, and be of lowest possible impedance (see 250.24(D) and 220.22).

I also feel the removing the grounded conductor from the main disconnect operation is important (even with MBJ connection to the conductor upstream of the main disconnect), and proper white identification of the grounded conductor (none so at present), is important considering how confusing most workers find corner grounded 3 phase systems.