Hey Guys, Another interesting one!
Am installing two 230 Volt@ 10.0 amp single phase autoclaves to steralize glassware, accessories, etc. in a clean research enviromment and classroom. The manufacturer specifies that GFCI Breakers should be installed.
Let me know if my reasoning is accurate.....
I have to boost 208v to 230v to maintain the operational speed. So, I'll need to select 3 KVA transformers and put the breakers in the secondary for them to protect my people.
There is no general reason that use of an autotransformer rather than an isolation transformer should cause false tripping of a GFCI breaker, provided that the wiring is done correctly. However, in your case we need to confirm that you intend to get the 208 volts as a line-to-line connection in a 208Y/120 service.
If that is the case, then you have some additional issues to deal with:
1. You can derive the 240 line-to-line by using an autotransformer connected to each of the line-to-neutral connections (boost autotransformer configuration.)This would create a greater than 120V output to neutral voltage such that the vector difference is 230.
If you do that, however, you will need to use a two pole GFCI, with a neutral feed through connection pigtail and terminal since the boost transformer primary will be a line-to-neutral load.
You would not have that particular requirement (the neutral included in GFCI) if you used a purely line-to-line connection to the autoclave.
2. Or you could run a boost transformer off the 208 volts line-to-line and use it to create a wire which is not in phase with or the same voltage as either phase line. In that case, the only current would be in the two phase lines and a simple two-pole GFCI without a neutral would work.
3. Or, you could do as you propose and create a new system voltage of 120/240 3 wire all derived from one phase of the system wye. Only one breaker then.
In all cases, however, there is a possibility that the capacitive coupling from the two line inputs to the heater winding could create a net current in the EGC. That was discussed in a thread a few months ago. By applying out of phase 120 volts to the two heater terminals the capacitive leakage currents should cancel out well enough that the GFCI will hold. That argues for solution 3, which could be implemented with either an autotransformer or an isolation transformer. But not a low voltage boost transformer.
