Fundamental's of Contact resistance testing

 


Contact resisting test is a kind or commissioning, or maintenance test done to measure the resistance of an electrical connection. The connection could be pressed together, bolted, soldered, clamped or welded. Contact theory assumes that at some scale the metal surfaces are never completely flat.

The main purpose is to confirm the integrity of an electrical connection and catch problems very early if one exists. The Contact Resistance measurement can detect the following problems loose connections, bolted joints that are not adequately tensioned, corroded contacts surfaces and eroded/pitted contact surfaces. 

This test is usually carried out on, circuit breakers, contactors, relays and switches in closed condition to verify that the contacts make properly and there would be no heating on load. It is also carried out on cable terminations, joints, and busbar sections for the same reason listed above.

In the case of circuit breakers and fused contactors, they experience mechanical wear and tear on their contacts due to the arc that develops on opening and closing of the contacts during breaker operation.  The integrity of the contacts in vacuum bottles cannot determined as they are encased inside the bottle but even if they were visible like those of an Air Circuit Breaker the eye is not a good gauge to ascertain the integrity of the contacts. 

Contact resistance measurement is a perfect for proactive maintenance and to prevent a catastrophic failure.

ACCEPTABLE LIMIT FOR LOW RESISRTANCE TESTING

A low resistance test is a measurement where the typical values expected is below 1ohm. Values as low as 0.1 micro-ohm could be read from some low resistance testers which is why we don't use normal multimeters for contact resistance testing.

 A maximum contact resistance of 20 micro-ohms is usually quoted as the acceptable limit for a good electrical contact point.  For example, if you are testing a MV circuit breaker from stab to stab you should be aware that there is more than one contact point, the stab is bolted to the circuit breaker, the movable contact is usually also bolted in so if you evaluate carefully there could be more than 4 electrical connections/contacts. In this situation, the minimum resistance you should accept is 80 micro-ohms.



HOW IT WORKS AND PROCEDURE

Our test is carried out using a Megger digital low resistance micro-ohmmeter. (DLRO10). It works using the four-wire (Kelvin Bridge) DC voltage drop method.

This method involves connecting two pairs of wires to the material under test:

  • The wires are C1,C2, P1 &P2
  • The Current wires (C1 and C2) carry the current (I) to the test piece.
  • The Voltage wires (P1 and P2) measure the potential difference (V) across the test piece.
  •  A known current (I) is passed through the current wires (C1 and C2).
  • The voltage drop (V) across the device or material is measured using the voltage wires (P1 and P2).
  • The tester knows how much current it injected into the test piece,
  • From Ohm’s Law, if we pass a current through a resistance we will generate a voltage across the resistance.  The voltage developed between the two points where the probes touch the test item is measured by the tester and the resistance (R) is calculated by the tester using Ohm's law: R = V/I.


 


Comments

Post a Comment

Popular posts from this blog

How to check continuity over long distance

Image
When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test. Let me use a simple picture to layout the nature of my problem. A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible.  The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB.   All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility 3 spare cables was laid underground but without tag
Read more

Making sense of a transformer nameplate data

Image
The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. Name Of Manufacturer Serial Number Year Of Manufacture Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the other sourc
Read more

Stall Rotor Protection Using Magnetic Overload Relays Oil Dashpot type

Image
  A Magnetic Overload relay works by detecting the magnetic field strength generated by the current flowing to the motor.  In most cases the relay input is a representation of the  current (through C.Ts)  flowing into the motor.   The Magnetic Overload Relay is built in such a way that the magnetic flux set up by current flowing through the  coil  drags (pulls) the core upward. A typical dashpot type magnetic overload relay is shown in the image below.                                    Construction. The relay consists of two main parts which is the trip mechanism at the top and the dashpot at the bottom.  The parts of the relay are further divided into a coil, a plunger (core), an oil-filled dashpot and switching contacts (trip contacts). The plunger is attached to a piston (disc). The piston is suspended in an oil filled chamber.  The relay coil is connected in series with the motor supply circuit. During normal operation magnetic flux induced by the coil is not great enough to c
Read more