ECN Forum Forums General Topics of the Trade General Discussion Area Fictional Scenario Discussion

Hi everyone, and thanks for the interest on this thread. I'll create another scenario this weekend and post to this group, possibly to the Mike Holt group also ??

Anyhow, the reasons of what I based these scenarios on are listed below:

Situation 1: This would cover the fundamentals of how a grounded conductor works, along with the infamous 240/120 VAC 1 phase 3 wire system. From this part, I was hoping to explain that the multiwire transformer secondary is primarily 240 volts, with the "neutral" coming from the center tap , meaning that the nominal voltage is 240 volts, but is cut in half when using the center tapped neutral conductor.

On the same subject, the conductor that gets grounded has no bearing on system operation, only on the maximum voltage to ground.

I have heard people insist that this connection would require a derated transformer, others arguing that this would short out the transformer, still others saying that nothing would work at all!!

This part is a basic theory area.


Situation 2: This was intended to explain that even though there is no physical connection of the system to the earth at any point, the system will still operate correctly. The hazard of ground fault to personnel touching a metallic enclosure that is connected this way can be low or high, depending on if there is any Capacitive Coupling between the system and the transformer's secondary. I thought this would cover the reasons why an earth bond is needed.


Situation 3: This one was added to cover the reasons for having the earth connection and grounded conductor at both the transformer and the service. I have seen older open delta "V" connected 3 wire systems that have only the center tap on one transformer grounded at the pole, then the service equipment is grounded at the main. No grounded conductor was brought to the service from the pole.
This one has odd voltage readings and most certainly will be a hazard for any short to ground.
The G.C. I work with has this for the old shop in the back of the Office. This property was bought about 4 years ago. We designed a new Office renovation on the property, but left the existing shop as is - including the separate-existing 3 wire delta service for it. Prior to this location being completed, the Office was at another location [this was the third new office in 10 years].

When I was designing the Electrical systems and such, I was planning to connect the location to a newly installed and dedicated 75 KVA 208Y/120 VAC 3 phase 4 wire Wye system. The existing power was on a 50 KVA 1 phase xfmr, and the open delta was two 5 KVA xfmrs - total of 8.3 KVA, but only 3 wire.
When I met with the SCE design Engineer to discuss this, we both took a good look at the existing delta system. SCE wanted to remove as many loads from the existing 4 KV primary feeders as possible, and we were trying to figure a way around the cost of a new 4 wire Wye system to the customer. As we were studying the pole, we both simutaniously spotted the point of grounding!! Was quite funny, so that's the basis of the 3rd situation.

As for situation 4 [Lightning], it will work out better for the first situation. One reason I eliminated the connection of the cold water lines wasn't to be cute , but to limit the influences of surges sneaking in through the city water. In this case, the surges will be spread across the building's foundation and limited to intensity due to the fact that the flow is restricted by a somewhat centralized spot of concentration.

The amount of flow that is added to the discharge is increased by a direct connection of the electrical system to city water. The surges can easilly connect through the primary feeders, flash across the transformer[s]' core and coil, and down through the secondary feeders. Positive charges can be brought up easilly from a multitude of locations.

The least protected system would be the one that has no earth grounding to the system [it is more hazardous to personnel], whereas the one grounded only at the pole would have the transformer be the "sacrificial" component and would most often cause high surge values to the secondary - creating further hazard to personnel.

So to end this, I am glad to see some interest here. We can discuss these things more indepth later, and/or in another thread.

I'll come up with some more topics that would [hopefully] contain an equal amount of theory, code and design issues.

Any ideas?? send them my way!!


Once again, Thank you for the interest!!


Scott "S.E.T."