Suppose the instrument is rated at IP42. A moisture rating of 2 means 'dripping water'. Therefore, it would not be resistant to flying spray. An instrument which is used in an environment that exceeds its IP rating likely means that the user will need a new instrument very soon. What about a rating of IP40? A moisture rating of 0 means that the instrument is not protected against any liquid ingress. The following tables provide a guide to various IP ratings and what they mean to the user:Table 2: Ingress and access protectionFirst No.Description0Non-protected1Objects equal to or greater than 50 mmProtected against access with back of hand2Objects equal to or greater than 12.5 mmProtected against access with jointed finger3Objects equal to or greater than 2.5 mmProtected against access with a tool4Objects equal to or greater than 1 mmProtected against access with a wire5Dust protected6Dust tightTable 3: Ingress of liquids protectionSecond No.Description0Non-protected1Water dripping vertically2Water dripping, enclosure tilted up to 15°3Spraying water, up to 60° angle from vertical4Splashing water, any direction5Jetting water, any direction6Powerful jetting water, any direction7Temporary immersion in water8Continuous immersion in waterEvaluation / interpretation of resultsRepeatabilityA good quality low resistance ohmmeter will provide repeatable readings within the accuracy specifications for the instrument. A typical accuracy specification is ±0.2% of reading, ±2 LSD (least significant digit). For a reading of 1500.0, this accuracy specification allows a variance of ±3.2 (0.2% x 1500 = 3; 2 LSD = 0.2).Additionally, the temperature coefficient must be factored into the reading if the ambient temperature deviates from the standard calibration temperature.Spot readings / base expectations for readingsSpot readings can be very important in understanding the condition of an electrical system. The user should have some idea of the level of the expected measurement based on the system’s data sheet or the supplier’s nameplate. Using this information as a baseline, variances can be identified and analyzed. A comparison can also be made with data collected on similar equipment.As noted, the data sheet or nameplate on a device should include electrical data relevant to its operation. The voltage, current and power requirements can be used to estimate the resistance of a circuit, and the operating specification can be used to determine the allowed change in a device (for example, with battery straps, connection resistances will change with time). Various national standards provide guidance for periodic test cycles.The temperature of the device will have a strong influence on the expected reading. As an example, the data collected on a hot motor will be different from a cold reading at the time of the installation. As the motor warms up, the resistance readings will go up. The resistance of copper windings responds to changes in temperature based on the basic nature of copper as a material. A more detailed review of temperature effects is covered in the appendix. Using the nameplate data for a motor, the expected percentage change in resistance due to temperature can be estimated using Table 4 for copper windings or the equation on which it is based.Different materials will have different temperature coefficients. As a result, the temperature correction equation will vary depending on the material being tested.Table 4: Copper: temperature / resistance relationshipTemp ºC (ºF)Resistance μΩ% Change-40 (-40)764.2-23.632 (0)921.5-7.868 (20)1000.00.0104 (40)1078.67.9140 (60)1157.215.7176 (80)1235.823.6212 (100)1314.331.4221 (105)1334.0 33.4R(end of test)/R(start of test) = (234.5 + T(end of test))/(234.5 + T(start of test)Trending In addition to comparing measurements made with a low resistance ohmmeter against some preset standard (spot test), the results should be saved and tracked against past and future measurements. Logging of measurements on standard forms with the data registered in a central database will improve the efficiency of the test operation. Previous test data can be reviewed by the user, and then on-site conditions can be determined. Developing a trend of readings helps the user better predict when a joint, weld, connection, or other component will become unsafe, and make the necessary repairs. Remember that degradation can be a slow process. Electrical equipment faces mechanical operations or thermal cycles that can fatigue the leads, contacts and bond connections. Additionally, these components can also be exposed to chemical attack from either the atmosphere or man made situations. Periodic tests and recording of the results will provide a database of values that can be used to develop resistance trends.Note: When taking periodic measurements, the user should always connect the probes in the same place on the test sample to ensure similar test conditions.The following are several examples of where trending can help the user make better informed maintenance decisions:Circuit breakersAs noted previously, mechanical wear and tear on circuit breaker contacts, that reduces the area of the contact surfaces combined with sparking and / or arcing, will increase the resistance across the working connections. This condition will produce heat that can reduce the effectiveness of the circuit breaker. Periodic measurements will show the rate of increase of the contact resistance value. When these values are compared to the original manufacturer’s specification, a decision can be made to continue or repair. By tracking the trend of the readings, the user will get an idea of when the circuit breaker should be pulled for service before damage is done.Stand-by battery back-up systemsThe interface between the terminals and the straps on battery back-up systems is subject to chemical attack from the acid atmosphere, thermal changes due to the charging and discharge currents and mechanical stress from vibration. Each of these factors can cause the resistance bond to degrade, resulting in the potential for a fire at a critical power discharge (due to the hydrogen gas atmosphere). Battery systems require diligent attention, as replacement batteries are both expensive and not off-the-shelf items. A failure situation can result in a battery system being out of service for a number of weeks. Periodic measurements of the strap resistance will identify those bond connections that have degraded since the last test and corrective action can be planned. Note: When connections have higher than normal resistance measurements, the user should not retighten the bolts, as this will over stress the soft lead connection. Over tightening does not cure the problem. The correct procedure is to disassemble the straps, clean, grease and then reconnect with the bolts tightened to the supplier’s torque level. All the connections should be balanced within a narrow tolerance of ±10 to 20%.In these and many other systems, time lost to repair defective equipment may be small compared to the cost of having equipment out of service for weeks. Periodic tests can avert many problems. Analyzing data against past results and reasonable standards allows the user to select the time when corrective work should be done. www.megger.com 2120 A guide to low resistance testing www.megger.com
