I have several heater circuits on our machine, and I'm debating how to hook up the single-phase transformers. I'll need to have to have 2 or 3 transformers to handle the power required of the heaters.

What is the best way to arrange the transformers? I'm debating between two options:

1) I could wire each transformer as a separate single-phase circuit, fusing both legs of the primary and the secondary of each transformer. Thus, the heater circuits (consisting of SSR and heater) would be floating and fused on both legs.

2) I could wire 3 (or 2) transformers in a delta-wye, with a grounded neutral on the secondary, and then I would only require three fuses on the secondary; Each heater circuit (SSR and heater) would then only require a fuse before the SSR, as the other end of the heater is at ground potential.

I'm debating the merits of both options. Option 2 just seems more efficient to me, but option 1 I think is more common. I usually see 240v single-phase circuits floating and fused on both legs, but wouldn't it be possible for a ground fault to trip only 1 fuse and create a hazard? For option 2, is there any potential problem with having a 240v single-phase circuit with one wire being a grounded neutral?

 

With a floating secondary, a ground fault wouldn't blow any fuses. Indeed that is an advantage of the grounded neutral scheme, a ground fault would more likely take the coil out of service. After innumerable headaches trouble shooting systems where various supplies are left floating with no reference to ground, I would lean strongly to having a grounded neutral. I _always_ ground DC power and transformer secondaries simply to have predictable input conditions for the connected gear and to avoid stray potentials. On an ongoing problem, I have a 24V supply that occasionally has 185 VDC to ground.

The machine works fine, but someone could get killed assuming low voltage. When we can afford downtime the negative side of the 24V will be bonded to the frame ground and we'll watch to see what smokes.

Regards
cww

 

For single-phase two-wire transformer secondaries, the NEC permits the primary overcurrent protection (OCP), up to 125% of rated primary current (VA rating divide by voltage), to serve as the protection for the secondary... technically both the secondary itself and the secondary conductors if of an ampacity not less than the primary OCP rating times the voltage ratio (2:1). You would only need to add secondary OCP if the factored primary OCP rating does not provide adequate secondary conductor or load equipment protection.

If done per above, you cannot configure the transformer primaries for delta or open delta supply (3-wire)... each must have a 2-wire supply and each ungrounded line fused.

Under the NEC, grounding one line of each transformer secondary is recommended but not required... but will require ground fault detection if not grounded. If grounded, a grounding electrode conductor (GEC) must be run from the existing grounding electrode system to the grounded terminal if you do ground the secondaries. In many cases, the GEC and equipment grounding conductor can be the same conductor provided the requirements for both are met by this one conductor [consult an EE or qualified electrician for details].

If do wire the primaries delta or open delta, you essentially reduce the primary OCPD's from 6 or 4, to 3. But then secondary conductor protection will be required so the number of OCPD's goes back up (but does permit higher primary OCPD rating, up to 250% of rated primary current). Grounding the secondary is still per above.

Another option you may want to look into is using a three-phase delta-delta transformer. Three-phase 3-wire secondaries have the same requirements as single-phase 2-wire secondaries [consult an EE or qualified electrician for details on grounding the secondary either as high leg or corner grounded].

 

So NEC requires a GFCI if the transformer secondaries are floating and ungrounded? Even if I fuse both legs?

I'm no legal expert, but I've seen a fair amount of 240v single phase circuits, and they are always fused on both legs. I don't think I've ever seen an industrial machine that actually had a GFCI installed. Although I suppose that's because the service transformer usually has a grounded center tap for two 120v circuits.

As for grounding one of the hot legs on the secondary, and omitting a fuse, I think there is actually one good reason not to do that: If the machine is installed without a ground connection, it could definitely create a hazard.

So, if I install each transformer as a single-phase transformer, each with 2 fuses on primary and 2 fuses on secondary, do I need a GFCI? Should I ground the center tap on each of the (2 or 3) transformers?

 

Not GFCI... GFD: ground fault detector. In its simplest form the device simply indicates if one leg is shorted to ground and should be remedied as soon as possible.

A floating (aka ungrounded) secondary with all legs fused will not blow any of the fuses on the first short to ground, or even subsequent shorts to ground of the same leg. The shorting of one leg to ground, no matter how well shorted, is no different than intentionally and solidly grounding that leg... and as one would suspect, you cannot ground two legs without letting the smoke out somewhere ...unless fused or otherwise protected from overcurrent.

You are correct that most 240V services are grounded. As for fusing one leg, grounding the other leg of three single secondaries is no less safe than having a 480Y/277V 3-phase 4-wire system. If the primaries are connected to "rotated" phases (i.e. AB, BC, CA), you will have essentially a 416Y/240V 3-phase 4-wire secondary. The power company does this all the time (though the primary may be wired wye) for small 3-phase services (three pots on a pole).

You previously didn't mention your secondaries had a center tap. The center tap isn't required to be grounded unless it is used as a circuit conductor... but I recommend grounding there and fusing both legs if that is your preference.