Why insulation test voltages are increasing

8 October 2018


In 2000 (and other times since), the IEEE revised its standard for insulation testing of motors, recommending a 10 kV test for equipment operating at 12 kV and above.

The need to update the IEEE standard is a response to the evolution of insulating materials. In the ‘60s, manufacturers developed XLPE (Cross-Linked Polyethylene) and EPR (Ethylene Propylene Rubber) insulating materials. The XLPE was widely hailed, to the extent of promising to last ‘forever’. But by the mid-‘70s, there was trouble in paradise.

Utilities began reporting unforeseen problems with some new synthetics… most dramatically illustrated in ‘treeing’ effects. Also called ‘water treeing’ because it is initiated by water molecules in the insulating material. In essence, a carbonized burn track gradually spreads and branches within the insulating material, resembling a tree under a microscope. Eventually, this ‘tree’ breaks through to ground and the cable faults.

The need to stop this disintegration in insulating material has led cable manufacturers to ongoing development of ever more complex polymeric materials - giant molecules with a regular pattern of cross linkages to create a matrix seemingly impermeable to wandering electrons.

Developing new standards from real test data

The formulae and extrusion processes of these materials are guarded like Emeril’s secret recipes, and so the IEEE requested, and has been gathering, field data from tests ever since. Improved extrusion processes mean less contaminants in the insulating materials. More complex formulae mean more cross linkage of giant organic polymers.

Both of these improvements reduce the response that the insulation exhibits toward a dielectric strength test. Absorption, the process by which insulating material realigns on the molecular level against an applied DC voltage field, has been reduced by these cross-linked molecules and a reduction in foreign matter.

Test voltage goes hand in hand with measurement range. As higher and higher voltages are required to pull smaller and smaller leakage currents (we’re talking nano-amps here), which translate into readings now going into the Tera-Ohm (ten to the 12th power) range. Old-style testers with much less measurement range are still fine for determining ‘good’ versus ‘bad’ insulation. But an ‘infinity’ reading doesn’t give a complete picture of the equipment’s condition and expectancy. For that, high range is necessary. Higher range demands higher voltage.

Staying ahead of the curve: Testing at 15kV

Keeping up with essential developments in field testing, Megger now offers the MIT1525, a model that is ahead of the curve with up to 15 kV of test voltage and 30 TΩ of range. But it’s not a single-feature model. The MIT1525 offers all the proven safety and convenience features for which Megger is renowned…CAT IV and fire-retardant dual case protection against arc flash, selector switch operation, automated test functions including ramp test, rapid charge of test item, and much more.

Making the investment in an instrument that is ahead of the standard is not only cost-effective as requirements continue to increase into the future, but also an important step to ensure you’re not behind the curve if new problems arise. Your MIT1525, offering 15 kV of test voltage, might be able to pick up on potential future problems other instruments might not discover; allowing you to provide essential data to keep the entire industry on track.