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Low Resistance Ohmmeter or Winding Resistance Test Set … What’s the difference?

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Protec Equipment Resources has a large available inventory of low resistance ohmmeters(10A, 100A or 200A models) and winding resistance test sets from leading manufacturers such as Vanguard and Raytech.  We offer technical guidance to our clients to assist in selecting the right tool for the application at hand.

When looking at low resistance ohmmeters and winding resistance test sets, at first glance these two test sets appear very similar.  After all both sets are used to measure very low resistances, right?  Well it is true that both test sets use an applied DC current to measure some very low resistances, but that is where the similarity ends.  Low resistance ohmmeters are well suited for a wide range of applications where measuring the integrity of a connection or bond is required.  In these applications there is no storage of energy in the test specimen.  When we look at the windings of a motor or transformer however, we must keep in mind that these windings are essentially large inductors.  The higher the voltage and the larger the (MVA) capacity, the higher the induction and hence a potential hazard when applying DC test current.  Standard low resistance ohmmeters simply do not provide the added safety features offered in a dedicated winding resistance test set such as the Vanguard models WRM10, WRM40 or Raytech models WR50 and WR14 units.

There is a potential hazard of DC test current applied to a large inductive load.  Should the test circuit become open while DC current is flowing hazardous voltages (possibly resulting in flash over) will occur. Care must be taken to ensure the test circuit does not accidentally become open:

• Ensure the test leads are securely attached to the winding’s terminals.
• Do not operate any instrument control which would open the measured circuit while DC current is flowing. Discharge the winding first.
• Do not disconnect any test leads while DC current is flowing. Ensure the winding is discharged first.
• When terminating the test, wait until the discharge indicator on the winding resistance test unit goes off before removing the current leads. When testing larger transformers it may take 30 seconds or more to discharge the winding. If a longer time (30 seconds plus) is required to charge a winding when the current is initiated a corresponding longer time will be required to discharge the winding.

Failure to take appropriate precautions can result in hazardous potentials which could be harmful to both personnel and test equipment.  A dedicated winding resistance test set is designed to minimize this hazard.

When using a dedicated winding resistance test set users and test equipment are protected by the shutdown circuit safety feature built into these test sets: any inadvertent disconnection of a test lead or loss of power to the instrument will safely discharge the energy stored in the test sample.