Power supply issues


Thread Starter


Power supply issues.

Panel of Yokogawa CS1000 system.
Power supply: 110 VAC from a UPS has floating neutral.
Three wires are terminated in the panel.
two of them carry the AC supply the third is the earth.

Problem: on checking the voltages between individual AC supply wires and the earthing wire it was observed that two different voltages exist, 48VAC and 64VAC. of course between the two supply wires the voltage is 112VAC.

is this imbalance normal or does it have any ill effect on the system.


Steve Myres, PE

In my experience with UPSes, the output neutral is bonded to the input neutral, so that if the input neutral is connected to the ground, so will the output neutral be. Check to see if your UPS should be operating in the fashion I have described, and if so, verify your feeder neutral is grounded.
this inbalance in power is not healthy. even we had this problem in our PLC panel and this thing ended up in the failure of the PLC. take some immediate corrective action in this regard.
This illustrates a common invisible problem in power source problems. Power transformers are commonly connected in Y-configuration in which the common point is usually grounded. However, three-phase transformers may also be
connected in Delta-configuration in which there is no ground. Delta transformers should only be used to drive full three-phase loads such as three phase electric motors. If any of the phases of a Delta-connected transformer are used to drive additional single phase loads, such an imbalance can easily occur. When unbalanced, the other phases will inherently adjust their voltage in response to the load and can damage any equipment attached not protected by individual circuit breakers.

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This is more tricky than knee-jerk "Oh, just ground it" responses may imply. "Scope of Supply" and supplier/department politics also may come into play (ie: Yokogawa warrenty likely depends on NOT touching this issue). Yes, there are also safety issues - I worked on one project with such a UPS and the metal cases of cheap data comm product had a 75v ground potential difference to the panel! Fortunately they all sat on rubber feet & weren't part of my scope of supply, but it still gave a bad "tickle" if you accidently touched any of them. I learned not to touch them.

Only safe/minimum-change solution (without a system overhaul) is to make sure ALL wires entering/leaving the "system" powered by the UPS also have floating earth. The problem is many AC/DC supplies will reference their signal-ground and/or 0v to "N", so in your case this may mean an RS-232 signal leaving the panel could have a +36v/+50v signal (with reference to a PC plugged into the wall sockets) instead of +/-12v.

You'll need to make sure any data communication lines are isolated - optically or by transformer. RS-232/422/485 isolators are widely available (such as from www.robustdc.com & others) and Ethernet UTP alwasy has transformer isolation. Any digital signal will need interposing relays. Any analog signals will need galvanic isolation of some sort. Personally, that's the "Proper" way to design things anyway - best that any signal leaving a panel make NO assumptions about the quality of the ground at the remote device.

best regards
- LynnL, www.digi.com