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· Registered
1 Posts
Discussion Starter · #1 ·
I Have A Three Phase Delta-delta 480vac To 240vac Transformer. This Transformers Has Me Concerned Due To The Voltage Readings To Ground. This Transformer Feeds A 3 Wire 240vac Panel.

When We Read Phase To Phase It Reads 240vac. I Measured Phase A To Ground And Read 10vac. Phase B To Ground Was 240vac, And Phase C To Ground Was 240vac. Phase A Increased To 120vac While Phase C Decreased To 120vac. We Used Two Meters And Witnessed Simultaneous Voltage Change On Those Two Phases.

These Connect Numerous Loads Which Do Not Seem To Be Effected By The Voltage Fluctuating. Once In Awhile The Machines Blows An Incoming Fuse, But Does Not Trip The Breaker.

I Know That This Is An Ungrounded Delta-delta Transformer. Do I Need To Ground One Phase Of The Secondary? Is It Possible That The Transformer Is Breaking Down?

P.s. We Are Only Using Single Phase 240vac Loads And Three Phase 240vac From This Panel I Mentioned Above.:(

Thanks For Suggestions

· Bilge Rat
motors and controls.........
10,061 Posts
The reason you're getting odd voltages is because the xfmr secondary is not grounded, and thus there's no reference point. You're reading meter impedance, VERY high impedance ground faults (not to worry, they're so small there's no hazard), capacitance in the test leads, etc.

Does this xfmr have an X0 or X4 terminal? If so, it's the mid-point of one of the phases, and if you're going to ground it, that's the spot. Then you'll have a 240/120 3PH 4W delta system. There are thousands of these in operation. You'll read 120V to ground on 2 phases, 208 to ground on the 3rd, and 240 between any two phases.

If the xfmr has just X1, X2, and X3, then you'll need to study the system a bit more to determine if it should be grounded. If it is to be grounded, ground X2, and you'll have a grounded B system. Also known as a corner grounded system. There are not a huge number of these still in use, they were more popular years ago. Be careful using the grounded B with single phase loads as at least one of the legs will be 240V to ground. Usually it's ok, but this is one thing that needs to be studied.

I've seen many of these systems ungrounded, and other than a possible code violation, very few problems. I'd actually suggest that 'if it ain't broke, don't fix it'.

· Ax grinder
2,049 Posts
I've seen many of these systems ungrounded, and other than a possible code violation, very few problems. I'd actually suggest that 'if it ain't broke, don't fix it'.
If you have an ungrounded secondary of a transformer then you won't be able to trip an overcurrent device due to a ground fault on a single phase of the secondary. It would take a second ground fault on a different phase before you would be able to trip an overcurrent device. This can be a serious problem.

To have a true ungrounded system as permitted in the NEC you would have to install ground fault indicators on the system to alert someone of an initial ground fault so the person could clear the fault.


· Moderator
18,866 Posts
Reading voltages to ground do nothing but confuse people. However, you did realize the secondary was not grounded, which it should be. The XFMR needs to be grounded too.

What advantage does a delta to delta XFMR provide? Delta to wye is the way to go. Especially in an industrial environment. Many electronic devises such as AC drives do not operate well on delta derived sytems. Plus you get a true neutral.
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