Frequently Asked Questions

What are the recommended calibration intervals?
PEM recommend that Rogowski units are calibrated every 2 years. The calibration drift over this period is very small, as the sensitivity is defined by coil geometry and highly stable passive components. HOWEVER, If the coil has been subjected to extreme mechanical stresses or thermal conditions, the calibration drift can be much higher. It is difficult to quantify, but as a policy we recommend that coils used in harsher environments (mechanical and thermal) are calibrated every year.

Can all units be repaired?
Not all units can be repaired by PEM. In particular, we can only repair CWT units that were made after August 2002. This is due to the obsolescence of some components and changes in our own manufacturing processes. For confirmation, please contact PEM and supply the serial numbers of your units in question.

What are PEM’s Calibration Accreditations?
Please note that PEM is not an ISO17025 accredited company. However, all of our calibration measurements are traceable to ISO17025 standards, (that is, all reference equipment used by PEM is calibrated to ISO17025 and carries up-to-date certification). Where a UKAS certificate is required, further calibration measurements would need to be undertaken externally to PEM. However, this is something that we can assist with.

When should a damaged coil be replaced?
Where the coil has broken windings the unit becomes inoperable and replacing the damaged coil becomes the only viable repair option. However, even when the unit appears to be in good working order, if coil damage is evident then coil replacement should be considered for the following reasons:

  • When a PEM Rogowski unit is purchased as new, the coil insulation is guaranteed up to a rated voltage which is stated on the coil. If the coil is worn to the extent that the outermost layer of insulation is damaged, the insulation rating can no longer be supported.
  • Where a coil becomes deformed, its positional variation can deteriorate. It should be noted that when PEM calibrate a unit, the conductor is central within the Rogowski loop, therefore this position should always give the most accurate reading.
  • An accurate reading from a Rogowski coil is dependent upon closing the Rogowski loop with a minimal discontinuity (gap) i.e. where the loop closes at the clip together mechanism. If the clip-together mechanism is damaged to the extent that the loop cannot be fully closed, then coil replacement is recommended.

Why won’t my unit measure 50/60Hz?
All Rogowski units have bandwidth limitations at low frequencies, and some of PEM’s higher sensitivity units are not capable of accurate measurement at 50/60Hz. Bandwidth information is available on all PEM product datasheets. If in doubt about the suitability of a product for your given application, please contact PEM.