TN 18 How Fast is Instant Off

The instant off method of removing external IR drop in potential measurements involves interrupting the cathodic protection current.  This action produces an instantaneous voltage drop which is considered to be the external IR drop.  The potential measured immediately after this instantaneous drop is considered to be the “IR drop free” potential of the structure.  Clearly, this method only works with an impressed current cathodic protection system where all the rectifiers on that system can be interrupted simultaneously and there are no other sources of current flowing through the electrolyte.

An issue which should be considered when using current interruption for instant-off measurements is:   What is meant by instantaneous?  The answer is not simple since it depends upon the structure, the electrolyte and the method of interrupting the current.  Putting the answer in electrical terms, it depends upon the capacitance and the inductance of the circuit.  IR drop free measurements can be made microseconds after current interruption on small uncoated specimens in a low resistance electrolyte.  Large coated structures, such as pipelines, or high resistance electrolytes, such as concrete, usually require several hundred milliseconds or more for IR-drop free measurements.  Interrupting current on the AC side rather than on the DC side of the rectifier will increase the time delay because the circuit inductance is higher.

For situations where current interruption cannot be reliably used to minimize external IR drop error in potential measurements, cathodic protection coupons are frequently used.  These are small pieces of metal similar to the structure which are electrically bonded to the structure through a switch.  Measurements are made as above except that the coupon rather than the rectifier is momentarily disconnected.  These measurements are termed instant disconnect measurements in order to distinguish them from instant off current interruption measurements.

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