If tanning beds need 240 on a 208V system, then the proper way is to provide one buck boost for single phase and two for three phase in an open delta configuration. A buck boost is not the same as a delta wye hooked up backwards. One should not just hook up a delta wye backwards - the ground connection can cause both scary and not so scary, but still serious, issues and the fact it wasn't designed that way can trip the breaker. The only safe way to hook this guy up backwards is to basically hook it up as a delta delta. If you do that, then you have basically disabled the OCP for all practical purposes. Ground fault indication should be applied. 250.21 requires the ground fault indication. I am not finding that 250.20 prevents using the transformer in this way. However, I am finding lots of math that shows it should not be.
Overcurrent protection trips when there is overcurrent. However, in a delta system, the first ground fault causes the voltage at that point to go to zero. I = V/R. So, since V goes to zero, there isn't an overcurrent situation.
But it isn't that simple if you are in a 208V system. The transformer has references to the utility voltage. The 208V is grounded with a reference voltage. The transformer is creating 240V on the other side. It isn't really ungrounded. It is coupled by magnetics. So, the fault isn't really at zero. There is going to be voltage drop along the conductors and the ground grid is going to climb.
If you purposely corner ground that transformer, you are going to have voltage drop along the conductors and you are going to have different ground potentials. I would love for somebody to go out and measure this on an existing system.
Now there is a system that causes tingles.
Buck boost transformers don't have an air gap. (or not a continuous one). They have copper to copper connections at various points. The secondary isn't really a secondary - it is connections on the transformer to the "primary" with short areas where the coils are different to boost it up. Copper to copper. Continuous system. Very different from a standard transformer.
Okay - this is what I've learned. Additional help in getting the word out and in figuring out how to fix the hundreds of wrong installations would be appreciated. I know an installer who has been doing this wrong for over a decade and who completes, by his own words, at least an installation a week.
Overcurrent protection trips when there is overcurrent. However, in a delta system, the first ground fault causes the voltage at that point to go to zero. I = V/R. So, since V goes to zero, there isn't an overcurrent situation.
But it isn't that simple if you are in a 208V system. The transformer has references to the utility voltage. The 208V is grounded with a reference voltage. The transformer is creating 240V on the other side. It isn't really ungrounded. It is coupled by magnetics. So, the fault isn't really at zero. There is going to be voltage drop along the conductors and the ground grid is going to climb.
If you purposely corner ground that transformer, you are going to have voltage drop along the conductors and you are going to have different ground potentials. I would love for somebody to go out and measure this on an existing system.
Now there is a system that causes tingles.
Buck boost transformers don't have an air gap. (or not a continuous one). They have copper to copper connections at various points. The secondary isn't really a secondary - it is connections on the transformer to the "primary" with short areas where the coils are different to boost it up. Copper to copper. Continuous system. Very different from a standard transformer.
Okay - this is what I've learned. Additional help in getting the word out and in figuring out how to fix the hundreds of wrong installations would be appreciated. I know an installer who has been doing this wrong for over a decade and who completes, by his own words, at least an installation a week.