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Many cathodic protection (CP) technicians struggle with the proper way to read and interpret shunt measurements or readings. 

 The Basics

A shunt is a calibrated resistor of a known value, which is connected in series within an electrical circuit.  By measuring the voltage across the shunt (mathematical formula used to calculate the relationship between current, voltage, and resistance: Ohms Law), the magnitude of current flowing within the circuit can be calculated.

Because we cannot physically see volts, amperes or resistance, a comparison to something familiar like a water/hydraulic circuit, is being used to demonstrate the relationship.

The National Electrical Manufactures Association (NEMA) has designated electrical enclosures into different categories or types depending on the level of protection required. These enclosure ratings affect a cathodic protection (CP) design or installation. 

Common rectifier problems are often misdiagnosed.  Despite zero output current while the rectifier indicates some level of output voltage, it is likely that the rectifier is in working order.  A broken or “open” cable or connection in the positive (anode) circuit and/or the negative (structure) output circuit could be the problem. 

Determining the Source of the Problem

Failures in the DC output circuit are rare.  When they do happen, they can occur suddenly.  One day the system is working fine and the next day you have an open circuit.  The following are check points:

• With a portable multimeter, confirm that there is some level of output voltage. Rectifier meters can “stick”, providing incorrect information.
• Check the millivolt drop on the rectifier current shunt. The ammeter could be defective while everything else is okay.

Poor or defective cathodic protection electrical connections can be the cause of issues in anode systems, rectifiers, test lead cables, test station connections and more.  Many of these issues can be avoided.

Types of Electrical Connections

An electrical connection can be defined as connecting two or more metallic conductors together to establish continuity between the individual conductors. The purpose of an electrical connection is to establish low electrical “contact resistance” between the conductors.  There are two types of cathodic protection electrical connections.  Each uses a different method to achieve the proper connection.

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%)

Situation

A new, deep well anode CP system and a standard, air-cooled CP rectifier were installed on a pipeline.  Prior to installation, current requirement tests were conducted.  Test results indicated that rectifier output of 36 to 40 DC amperes would be required to achieve proper protection levels.

Hardware Description

• 450’ deep anode well with 20 cast iron anodes in coke breeze
• New CP rectifier with a DC output rating of 50 volt and 50 amperes with a “standard” voltage adjustment tap configuration of 3 Coarse and 6 Fine settings
• AC input rating of the rectifier is 115/230 volts, 1 phase, 60 Hz

Understanding Rectifiers

Most cathodic protection (CP) rectifiers produced today are extremely reliable.  Yet, like any manmade device, failures can and will occur.  For CP rectifiers, in some instances, they will continue to operate despite component failure.

Most CP rectifiers in service today are air-cooled, single-phase transformer units. While they may appear complicated to provide needed DC current for cathodic protection to a steel structure, there are only two components required to have a fully functional rectifier:

1. Step-down transformer - to reduce the incoming AC power to a lower, usable voltage
2. Diode or rectifier stack - four diodes configured into a “full-wave bridge” rectifier stack

As in any industry, accurate communication is key to minimizing errors in system designs and material supply.

Take pipes. Cathodic protection (CP) systems are designed for many different types and sizes of pipe. In the design, it is common to specify certain types and sizes of pipefittings, such as an insulated pipe union, in order to produce an effective and efficient CP system. To accurately measure and order pipes and accessories, it is important to follow ANSI Pipe Schedule Standards.

Magnesium Anode Current Output & Service Life

Key Facts:

• Longer anodes produce more current than shorter anodes.
• Doubling the diameter of the anode will increase the volume (weight) by a factor of 4; yet, will only increase the current output approximately 15%.
• Greater anode weight (volume) increases the anode or system life.
• Surface area of the anode is not as important as some people believe.
• High potential anodes will always generate more current than a standard potential anode given the same resistance-to-earth.