24VDC Floating Power Supply and loss of xmtr readings


Thread Starter

Bobby Chan

hello good folks,
I am currently trying to resolve a problem with transmitter readings wrecking havoc on my process, I Measure this power supply to ground,
at different times, Measurement #1: 24V on dc+ and near 0V at dc-, Measurement #2: dc+ at 0V and dc- at 24VDC-.(Please excuse my lack of experience with floating DC systems). Could this be an electrical problem with the power supply?.
Is it reasonable to expect measurement #2, if a wiring error causes the dc+ somehow become grounded? I suspect, that when this voltage bumps, it is causing problems with the transmitter 4-20 out.

related information:
1. While checking the wiring on one transmitter, with the dc+ fuse (to the
transmitter) out, the DC- touched the metallic transmitter housing, and i observed an spark.
2. using an ohmmeter, measurement #1 results a near 0 ohm reading to ground on 24VDC+, and high ohms on 24VDC+
3. using an ohmmeter, measurement #2 results a near 0 ohm reading to ground on 24VDC-, and high ohm on 24VDC-.
I also speculate that some device action may be the result of this grounding measurement at the power supply.

4. If i measure other power supplies in the system to ground,(also not grounded at 24Vdc-, they always measure 24v+ on DC+ and near ground potential at 24vdc-.
5. transmitters are Rosemount smarts(model 3051).
Any suggestions will be graciously accepted..and thank you in advance

Nick Carlin/ B.C.

A Aziz b Ahmad

> hello N.C/B.C

Our ESD system experience more or less the same situation as what you has described.

When we connected the AI card to the 24Vdc floating power supply ( +12 and -12Vdc) the reading we get at our MMI is only half of what we injected from the field. We solved it by installing seperate power supply and separate grounding.