EXPERT TIP #5: CHECKING/TESTING CP RECTIFIER DIODES
Cathodic protection Power Supply or Rectifier problems are the cause of Cathodic Protection (CP) system failures, 58% of the time. When troubleshooting a failed CP rectifier, the most common problem occurs with the rectifier stack. (Rectifier failure causes: Rectifier Diode Stack – 85%; Meters, Breakers, Fuses – 12%; Transformers, Chokes – 3%)
Stack Types
Conventional transformer type rectifiers have a single-phase, full wave silicon stack with manual tap control. Selenium stacks require a different test method.
Diode Functions
A diode is essentially an electronic “check valve”. This value allows current to flow in one direction and block it from the opposite direction. The 
symbol is the common symbol for a diode. As shown here, current can flow from left to right but is blocked from right to left.
How to Test a Diode Outside of a Rectifier
Most Digital Multimeters (DMM) will have a “diode test” mode. It may share a space on the dial with another function. In the diode test function, the meter will provide a small, fixed amount of current through the diode being tested. The displayed value on the DMM is the voltage drop across the diode.
- Be certain that the rectifier power is “OFF” and all safety precautions and proper PPE are utilized.
- It is critical that the diode be disconnected from the circuit to avoid parallel conductive paths. This may require disconnecting one end of the diode from the circuit. Methods specific to testing Rectifier diodes are discussed below.
- Connect the DMM test leads on both sides of the diode and record the measurement displayed on the DMM. Then, reverse the test leads and record the measurement displayed again.
- A good diode will read from 0.450 to 0.800 volts “drop” in one direction and “open circuit” in the opposite direction. “OL” is the typical DMM display for open circuit condition.
- When a diode fails, it will either fail in a shorted or open circuit condition.
- A “shorted” diode will display 0.000 volts in both Farwest Expert Tip 5 Checking CP Rectifier Diodes 060820directions
- An “open” diode will display “OL” in both directions.
Testing Diodes in a CP Rectifier from the Front Panel
Individual diodes can be tested from the rectifier front panel. This is a good option because getting physical access to the diode stack (as discussed above) can be challenging. In most air-cooled rectifiers, the diode stack assembly is located low and to the rear in the enclosure. With oil-cooled rectifiers, the diode stack is submerged in oil and rarely accessible without pulling the component rack up and out of the oil, which can be messy.
Preparation for Testing
The diodes to be tested must be isolated from other circuit components. With a CP rectifier, this would include the main transformer, the anode or the structure circuits. The steps for isolating these components include:
- Verify that AC power to the rectifier is “OFF” and power is locked and tagged out.
- Remove one of the voltage adjust tap link bars (coarse or fine) on the front panel to isolate the transformer.
- Disconnect either the positive or negative output cable from the output lug.
- Ensure that the AC secondary breaker is “ON”. If a fuse is utilized instead of a circuit breaker, ensure that the fuse is installed and in working order (not “blown”).
Testing the Diodes
Set the DMM to the “diode test” function. Ensure that your test lead cables are in good operating condition.
Refer to the test diagram and chart (below). A letter identifies each individual diode, as well as the test points:
- Diodes - A thru D
- Test Points - E thru H.
To test diode “A”:
- Make contact to test points E & H with the DMM test leads. Read the DMM display and note the values.
- Reverse the test lead polarity on test points E & H. Read display and note the values.
Diode “A” is in working order (good) when the reading is between 0.450 - 0.800 in one polarity and OL (open circuit) in the other polarity. A shorted diode will read 0.000 in both directions.
To test diodes B, C & D, reference the chart below.
This method allows the testing of each individual diode without the need to directly access the physical component. This is a real benefit when troubleshooting crowded air-cooled or messy oil-immersed rectifiers.
CAUTION! Most silicon stacks will have surge arrestors installed in parallel with the AC input and DC output of the stack. The arrestor may be mounted directly on the stack or separately on the side of the stack. If this is the case and your tests indicate two shorted (bad) or more diodes, it is possible that the diodes are in working order (good) but the surge suppressor is shorted. At this point, you have no choice but to find a way to access the physical components for individual diode testing.