ZP O2 Probe Troubleshooting Tips For PCC-3, DCS-3 & BMUFILED UNDER: BMU, PCC-3, PI Panel
When working with ZP O2 Probes as a way to avoid sending back equipment under an RMA that is in good working condition when the problem could actually be an installation / field issue, here are a few things to check out.
The PCC Z board has Diagnostic codes that can be read via the Examine menu , or by counting the LED blink code on the Z board.
The BMU has virtually the same diagnostics, but displays the result as a text message on the LCD keypad via: MENU / UTILITIES / OXYGEN CELL STATUS.
The ZP diagnostic codes/messsages (see pg 16 ) cover MOST of the field wiring errors that can be made, but they DO NOT COVER ALL POSSIBLE field wiring errors!
Cell T/C (thermocouple) wirng erros are completely covered with shorted, open, and reversed diagnostic messages.
However, the Cell wiring only has an Open wiring diagnostic.
You are forced to use your deductive powers to diagnose field wiring problems related to cell wiring.
There are technical reasons why it isn't feasible to give diagnostic message for all cell wiring conditions.
Steps to Troubleshoot
Reversed Polarity Cell wiring does NOT have a diagnostic code/message. The main symptoms are:
1) With burner off, and stack purged, the ZP displays something close to 20.6% O2, as it should because that's the normal ambient O2 level. see chart on pg 21, 20.5% O2 is approx 0.0 mV, so +/- polarity doesn't matter (0.0 = 0.0).
2) When the burner is firing and the O2 in the stack is less than 20.6%, the ZP displays an O2 reading higher than 20.6% O2; assuming the ZP is otherwise operating normally (Cell at 800C and properly calibrated).
Look at the chart on pg 21, if the cell wire polarity is backwards, it woould apply a negative voltage to the Z board (or BMU) and thus would display a higher and higher percent O2.
Have the User re-verify the field wiring, and remind them that the ZP terminals are NOT in numerical order. Terminal numbers are molded into the circular terminal block in the ZP head. From the top going clockwise the sequence is: 1, 3, 4, 2, 5, 6
Shorted Cell field wiring does NOT have a diagnostic code/message. The main symptom is:
This causes the ZP / BMU to always sense 0.0 mV. From the chart on pg 21, you can see that the ZP will always display ~20.6%, regardless of the O2 in the flue Gas (Assuming the ZP is at 800C and the calibration constants are correct)
Cell field wiring Open ...OR...High Combustibles diagnostic code/message:
There is an internal pull-up resistor on the Z board, and in the BMU, (see pg 5, top). If the field wiring is open circuit, the electronics will read a voltage higher than ~0.5 V (or 500 mV), and possibly as high as 1.5 to 2.5 V.
If the burner is operating poorly and is making substantial amounts of CO or Combustibles, the 800C (1472F) cell temperature will burn the CO or Combustibles in the ZP Cell cavity and consume all of the oxygen on the Flue Gas side of the Cell.
Looking at the table on pg 21, you can see that the Cell mV rapidly increases as O2 drops to zero and or goes negative. Therefore a high Cell mV signal can be either an Open wire OR High Combustibles/CO. The User should check the field wiring for opens and also get a portable analyzer to check the flue gas for high combustibles or CO.
Blown Fuse or no AC Power
The PCC / DCS Z board hardware can detect this condition and trigger this error code. The BMU can NOT DETECT THIS CONDITION.
For a BMU installation:
Ask the User to look at the MENU / UTILITIES / OXYGEN CELL STATUS screen on the LCD/keypad and tell you the Cell temperature. If it is near room temperature (25C) or near the stack temperature (100 - 175C), the heater probably isn't working. Ask them to check for 120 Vac on BMU terminals 79 to 80. Some users forget to connect 120 Vac to theses terminals, or have an external fuse.
If there is power on terminals 79-80, with the cell heater wires connected, measure voltage on terminals 81 to 82 (the power to the cell heater). If terminals 79 to 80 are 120 Vac, and terminals 81 to 82 read zero volts, the internal fuse inside the BMU is probably blown. We don't generally recommend field fuse replacement; however, a competent tech should be able to remove the cover and repalce the fuse near these terminals.
If there is power on terminals 79-80, and terminals 81 to 82 reads 90-120 Vac witha digital meter: check the heater field wires for an open circuit. If that's OK, turn off the 120 Vac, remove the heater field wires in the ZP head and measure the cell heater resistance in the ZP head, it should be 50-55 ohms (pg 21). If it is open circuit directly on the cell terminals with field wires removed, it is probably a bad ZP heater and the ZP should be replaced.