Help! My Resistance Reading Says “Hi Cap”. What Does this Mean?

24 May 2018


Operators using the new MIT2500 insulation tester often ask us why they get hi cap when they are simply trying to take a resistance reading.  This insulation tester focuses on the 1 kV (building wiring equipment) range of testing, but also allows the operator some extra maneuvering room.

The MIT2500 can stress test items at higher voltages, as well as provide the voltage to venture into testing medium-voltage equipment. But like any other electrical equipment design, there’s a tradeoff. It can be both interesting and practical to have a basic knowledge of why testers are designed and operate as they do.

Insulation testers for building wiring

Insulation testers for building wiring are typically rated up to 1 kV. In the working environment, small handheld models are the order of the day. When moving to medium voltage test items, the testers have bigger transformers and become a bit heavier and more bulky. This is understandable and pose no general problem. But conformance with IEC 61010 sets very tight standards for internal creepage and clearance distances so that the unit will not arc under external stress, like fault clearance, which could cause lethal damage. All Megger insulation testers provide maximum Class IV protection.

Testers are not dangerous, but the item under test might be

In any given insulation test, the tester itself is not dangerous; but the test item might be. That’s where IEC 61010 protects the operator. The test item can store a large static charge, depending on its design; however this is not a problem. The tester discharges the static charge automatically, without being ‘asked’. The operator needn’t perform any tasks for this to happen.

Testing above 1kV

When testing above 1 kV, a design conundrum presents itself. The tester has to meet tight creepage and clearance parameters, yet has to be held in one hand. The solution is to automatically inhibit testing above 1 kV on high capacitance test items that could store more energy than could be discharged while keeping within IEC 61010 rules.

The tester first measures the capacitance of the IUT. The limit for testing above 1 kV is 15 nF. That’s a lot of capacitance, but miles of cable can exceed it. The tester will advise the operator of Hi Cap, and inhibit the test, for safety. Should this occur, click down to 1 kV and you’re safe. If you must test at medium voltage? Get a medium voltage insulation tester (5 to 15 kV). They’re bigger, and therefore afford larger creepage and clearance that can safely discharge any static charge.