Monitoring Decouplers with the RM4211
The Bullhorn RM4211 was developed to be a low-cost, battery-powered, satellite RMU capable of taking AC and DC measurements on 3 high-voltage analog channels that fit into many applications. Typical applications for this product include bonds, mid-point test stations, and solar-powered rectifiers. A lesser-known application ideal for the RM4211's capabilities is the detection of decoupler issues.
Decouplers perform two electrical functions: DC isolation and AC grounding. They are used in the industry to isolate electrical systems that may compromise the effectiveness of cathodic protection (CP) systems. For example, safety grounding systems are necessary to protect personnel working on CP systems. Grounding systems that are electrically connected to a cathodically protected structure will short CP currents to ground, thus compromising the efficiency of the CP system. To keep CP protection on an asset, and off of a grounding system, a decoupler can be used to block the flow of DC current while allowing the passage of AC current to protect against faults or lightning strikes. Similarly, these devices can be used in AC mitigation systems to divert AC current from the pipeline to ground while keeping cathodic protection on the pipeline only.
Although decouplers are very rugged, they can fail. It is critical to know when these devices fail because the CP system may be compromised or there may be a safety hazard, and the effects may go unnoticed until the next time a facility is inspected.
The Technique for Monitoring Decouplers
A method for detecting when a decoupler is not functioning properly is to use a remote monitor. The RM4211 is perfect for this application because it is battery powered, has the analog inputs necessary to test for failure, and alarms can be set to notify if the decoupler has failed.
Decouplers have many applications; however, the typical wiring to the RM4211 can be observed in Figure 1 below.
• Channel 1 can be used to take an optional pipe-to-soil measurement between a stationary reference cell (negative) and the structure (positive).
• Channel 2 can be used to monitor the decoupler with positive and negative connections going to the respective inputs of the RM4211. Installing surge protection is recommended to protect the RMU should there be a surge event across the decoupler.
Since the remote monitor allows for 3 analog measurements, additional items can be monitored such as coupons, reference cells, and current transducers.
• If a coupon is used, it can be connected to Channel 1 which will allow an instant off measurement.
• A current transducer on Channel 3 can provide some insights as to how much current is flowing through the decoupler. It is important to note the scaling specifications of the current transducer and apply the appropriate scaling on the remote monitor to accurately report current. See Figure 2 for more information.
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RMU Channel Configuration
High and low alarms can be set up for the RMU via the mobile configuration tool to automatically send in a notification when a measurement is taken. Although the device can capture measurements every 15 minutes, it is recommended to capture a measurement daily to save on data costs and extend battery life. The thresholds users set will depend on the information needed. Assuming the configuration represented in Figure 1, channels can be set up as follows:
• Channel 1 DC (Structure): This will be a DC measurement representative of the CP system applied. No alarm is necessary unless a specific condition is trying to be tested.
• Channel 1 AC (Structure): No alarm is necessary unless a specific condition is trying to be tested.
• Channel 2 DC (Decoupler): Under normal operating conditions, a voltage across the decoupler is expected. If the decoupler fails (short), a DC voltage approaching zero is expected. A baseline measurement should be taken, then a low voltage alarm slightly above zero, but less than the baseline, should be applied. A decoupler that fails (open) will be less evident of failure because a DC voltage will still appear.
• Channel 2 AC (Decoupler): Under normal operating conditions, AC voltage across a decoupler is approximately zero. Should a decoupler fail in an open condition, AC voltage will typically increase.