How to test isolation joints?

Isolation joints are an important part of a cathodically protected pipeline. Verification of the joints should therefore be a part of the periodic cathodic protection system service control.

The purpose of the isolation joint is to prevent detrimental electro-chemical interaction and improve the effectiveness of the cathodic protection system. Isolating joints are also used to ensure effective current distribution for cathodic protection system.

Current passing through an isolation joint, as a result of insufficient isolation, may cause corrosion. Further, if current from the cathodic protection system is picked up by the copper grounding system, the current requirement will increase considerably. This may result too low cathodic protection current capacity and hence, reduced cathodic protection effect. Additionally, such stray current (leakage current) will make it more difficult to control the cathodic protection.

The below describe a couple of measurement techniques used to verify the condition of isolation joints.

Current leakage measurements

Current leakage measurements (detection) is performed by using a current transmitter and receiver. This technique is also called audio-frequency generator measurements. When the cathodic protection system is switched off, a coded current is impressed from a specially designed transmitter through one of the cathodic protection anodes to the cathodically protected pipeline. The coded current can be detected and measured by a receiver. No signal picked up indicate that the isolation joint provides sufficient isolation. For further information on the measurement technique, click here.

Line current measurements

Line current measurements are based on measuring the potential drop for a certain distance on the protection side of the isolation joint. This method requires an accurate low resistance ohm meter (micro ohm meter). No voltage drop indicates no current flow, and hence, sufficient isolated isolation joint. This test can also be done by using a battery or a DC source connected to both sides of the isolation joints, and outside the potential drop measurement connections.

Battery source and clamp ampere meter

A battery (or DC power source) are connected across the isolation joint with the positive output connected to one side of the isolation joint and the negative to the other side. A specially designed clamp-on current probe is used to measure any current flowing between the positive and negative power source connections. No current show sufficient isolation.

Reference potential measurements

By connecting a current interrupter in the cathodic protection anode circuit to switch on and off the cathodic protection, the on- and off-potentials can be measured on the non-protected side of the isolation joint. The difference between the on- and off-potential measured, indicate the resistance through the isolation joint. Less difference – higher resistance.