Open delta?

[vm3118]

We were troubleshooting an mcc in our training facility and got some voltage readings that im not quite sure what they mean. This is in an older elementary school and was adapted as a training facility.
As we checked voltage on MCC bucket showed
480 volts A/B 480 volts A/C 480 volts B/C
480 volts A/Ground 480 volts B/Ground 0 volts C/Ground

Any thoughts on this setup would be greatly appreciated

 

[don_resqcapt19]

That would be a corner grounded delta. It would not be a very common system for a school.

 

[roger]

It is a "Corner Grounded" Delta.

Click here for some more info.

Roger

 

[vm3118]

i will look into the corner grounded delta in the morning.

also was reading the forums and this morning we had a lightning strike right by the school so im wondering if we dont have an ungrounded delta system and a lightning resister might have gotten blown on the line side of the main

 

[LarryFine]

That would be a corner grounded delta. It would not be a very common system for a school.

It actually sounds likely if it's a Delta source, which means there's no 277v line-to-neutral lighting; it's all 120v.

 

[vm3118]

i believe all the lighting is 120 its looking like the corner grounded delta all the more
I really appreciate the insight

 

[LarryFine]

i believe all the lighting is 120 its looking like the corner grounded delta all the more

It never hurts to make sure the grounding is intentional. If you need to, you can look for a bonding jumper in the main disco enclosure.

In fact, the grounded conductor should be white, just like the familiar single-phase neutral, as well as grounded similarly.

 

[don_resqcapt19]

It never hurts to make sure the grounding is intentional. If you need to, you can look for a bonding jumper in the main disco enclosure.

In fact, the grounded conductor should be white, just like the familiar single-phase neutral, as well as grounded similarly.

Larry,
I know that the code requires that the grounded conductor of a corner grounded system but, I have never seen a white used for that purpose.

 

[cripple]

Open Delta?

Open Delta?

Your reading indicate that it is delta system, as to if it is an intentionally corner grounded system would require closure examination. The first thing to look for is ground detection system, which is required by code on all ungrounded systems. The second thing as noted is to look in the main service disconnect and see if there is a main bonding jumper. The main service disconnect would have a grounding electrode conductor connect from one of the phase to the building grounding electrode system and a main bonding jumper connecting the grounded phase to the service disconnect enclosure.
If it is an intentionally grounded system the grounded conductor is required by Article 200 to be identified with a white or gray outer finish or by three continuous white stripes, there are no exception.

 

[achieverelectro]

We were troubleshooting an mcc in our training facility and got some voltage readings that im not quite sure what they mean. This is in an older elementary school and was adapted as a training facility.
As we checked voltage on MCC bucket showed
480 volts A/B 480 volts A/C 480 volts B/C
480 volts A/Ground 480 volts B/Ground 0 volts C/Ground

Any thoughts on this setup would be greatly appreciated

If I saw those reading here on my ship, I'd be looking at a dead-ground on the C phase.
I find it rather hard to believe that in this day and age there are circuits that are engineered to dump 1/3 of your voltage right down the toilet, (so to speak). I guess the boys toting up the electric bills down at the utility don't mind it though.

 

[infinity]

If I saw those reading here on my ship, I'd be looking at a dead-ground on the C phase.
I find it rather hard to believe that in this day and age there are circuits that are engineered to dump 1/3 of your voltage right down the toilet, (so to speak). I guess the boys toting up the electric bills down at the utility don't mind it though.

Care to explain what this means?

 

[achieverelectro]

Care to explain what this means?

Sure. Aboard ship, we use a floating ground system, since it's illegal, vice the CFR 111.05-11, to use the ship's hull as a conductor,(except for certain limited circumstances).
Since I am, essentially, the Chief Cook and Bottlewasher of my very own utility company, from the generators,(4 auto-paralleling 6.6 kv turbo-diesels), right down to the light bulbs, I get to look at the entire picture.

Out here, deliberately "grounding",(actually "hulling"), one of your legs is also incredibly wasteful, not to mention destructive to things like intended sacrificial anodes and unintended anodes, (like where the different steels of bearings meet the mild steel of the hull, and welded seams of the plates,all immersed in seawater...BAD!).

On my installation, I have Bender Ground Detectors tied into the central engine alarm system, and am installing Erickson Ground Deteclor Lights at the Distribution panels,(it will ease my task of "chasing grounds"...plus, I'm a bit "old school"...I've sailed a lot of ships built in the 1960's, where every distribution and feeder panel had GD lights standard).

If I were to ground one of my phases, the Chief Engineer and the bean counters at World Headquarters would initially get all squiffy about the ship's increased fuel consumption figures. And then would flip right out at the next shipyard period from all the "change orders".

But even shoreside, if you think about it, the customer is paying for the current on all 3 phases, so to then take one of those phases and send it back through the earth to the opposite pole of the generator without it doing much, (if any), useful work...
I s'pose there are very good reasons for this, but it seems odd from where I float.

 

[roger]

I find it rather hard to believe that in this day and age there are circuits that are engineered to dump 1/3 of your voltage right down the toilet, (so to speak). I guess the boys toting up the electric bills down at the utility don't mind it though.

Huh, are saying that grounding or bonding a phase to a structure somehow negates it's voltage?

How do you think that a phase to phase reading even to the grounded phase still gives you the nominal phase to phase voltage?

The voltage is still there on the grounded phase.

Roger

 

[achieverelectro]

Huh, are saying that grounding or bonding a phase to a structure somehow negates it's voltage?

How do you think that a phase to phase reading even to the grounded phase still gives you the nominal phase to phase voltage?

The voltage is still there on the grounded phase.

Roger

When I'm chasing grounds solo, and I don't have access to GD lights, or another fellow with a radio gawking at the GD lights down below while I switch off branch circuits, I will usually open a panel and check each phase to ground.

(For simplicity's and familiarity's sake, say this is a 3 phase 440 volt system, floating ground).

On the phase that is grounded, I will see O volts...no potential difference between those two points, and the other two phases will show 440 volts PD to ground. All three phases will be showing 440 volts to each other.

Normally, what I SHOULD see aboard ship is each phase at 248-255volts to ground, and 440 volts to each other.

So yes, I guess the PD is still there between phases, but using the ground as the return path to the genny's pole is what is anathema to me.
Using this method, I could see wiring up a three phase motor with two wires from the ungrounded phases at the load side of the motor controller, and a pigtail from the hull to one of the motor terminal leads,(assuming a wye connected 220/440 vac motor, ie: 3 line pigtails in the peckerhead)...although I'd soon thereafter be marooned for incompetence and possible criminal negligence by doing so.

 

[don_resqcapt19]

If I were to ground one of my phases, the Chief Engineer and the bean counters at World Headquarters would initially get all squiffy about the ship's increased fuel consumption figures. And then would flip right out at the next shipyard period from all the "change orders".

But even shoreside, if you think about it, the customer is paying for the current on all 3 phases, so to then take one of those phases and send it back through the earth to the opposite pole of the generator without it doing much, (if any), useful work...
I s'pose there are very good reasons for this, but it seems odd from where I float.

There would be no increase in fuel consumption if you grounded one of your phases and there is no increase in electrical usage by grounding one of the phases. This is not any different from a standard 120/240 single phase system with the center tap grounded. Current does not flow in the ground path unless there is a ground fault, or multiple points of connection from the grounded conductor to the earth (or hull).

 

[don_resqcapt19]

...
So yes, I guess the PD is still there between phases, but using the ground as the return path to the genny's pole is what is anathema to me.
Using this method, I could see wiring up a three phase motor with two wires from the ungrounded phases at the load side of the motor controller, and a pigtail from the hull to one of the motor terminal leads,(assuming a wye connected 220/440 vac motor, ie: 3 line pigtails in the peckerhead)...although I'd soon thereafter be marooned for incompetence and possible criminal negligence by doing so.

The ground is not used as a return path in a correctly wired grounded phase system. The grounded conductor is that path. There is a system bonding jumper installed between the grounded conductor and the grounding electrode conductor at the first means of disconnect of the system. This bonding jumper does not normally carry current, but it is part of the fault clearing path when there is a ground fault.

 

[achieverelectro]

There would be no increase in fuel consumption if you grounded one of your phases and there is no increase in electrical usage by grounding one of the phases. This is not any different from a standard 120/240 single phase system with the center tap grounded. Current does not flow in the ground path unless there is a ground fault, or multiple points of connection from the grounded conductor to the earth (or hull).

I'd hate to be the one to explain why one corner of my delta-wired transformer secondary was bolted to the hull in light of this:

http://law.justia.com/us/cfr/title46/46-4.0.1.3.14.2.45.5.html

They WILL put you on the beach for that.

 

[roger]

So yes, I guess the PD is still there between phases, but using the ground as the return path to the genny's pole is what is anathema to me.
Using this method, I could see wiring up a three phase motor with two wires from the ungrounded phases at the load side of the motor controller, and a pigtail from the hull to one of the motor terminal leads,(assuming a wye connected 220/440 vac motor, ie: 3 line pigtails in the peckerhead)...although I'd soon thereafter be marooned for incompetence and possible criminal negligence by doing so.

Your case scenario would be no different than using an EGC as part of a two wire 120v circuit, it would work but it is not the correct way to do it and I would expect you to have to suffer the consequences but, that doesn't change the fact that a grounded (or bonded to a ships hull) Delta doesn't use or loose any voltage as far as being a three phase system. The grounded conductor is simply another phase conductor. The only thing that has changed is that you can throw away the GD.

Now, as far as other reasons not to bond it to a ships hull I'm sure there is.

Roger

 

[roger]

I'd hate to be the one to explain why one corner of my delta-wired transformer secondary was bolted to the hull in light of this:

http://law.justia.com/us/cfr/title46/46-4.0.1.3.14.2.45.5.html

They WILL put you on the beach for that.

Why do you think a ships hull would carry current if all the downstream wiring is connected properly.

It's illegal for us (land lubber sparkies) to use EGC paths for part of a circuit, this is the same as you using the bonded ship hull, I think this may be where you are confused.

As Don said, the bonded ship hull should not carry current if all the wiring is done correctly.

Roger

 

[mivey]

I started to post a long multi-quoted reply but think Don & Roger have covered the issues. I think this is just some fundamental misunderstanding of things on achieverelectro's part about using the hull as a conductor. This is also shown in another thread:

...Frankly, this drives me nuts as they can't seem to understand that they are using the hull of the mudboat as a conductor...