Low resistance testing works your way

1 March 2016

Electrical Tester

It seems a very simple idea – when a company is developing a new product, the first thing it should do is to ask potential customers what they want. Amazingly, this doesn’t always happen! Megger, however, is committed to producing instruments that accurately meet its customers’ needs. So, before the latest addition to the company’s very successful range of low-resistance testers was developed, many customers were consulted. Here’s what they wanted.

High power. A popular application for low-resistance test sets is checking the resistance of HV circuit breaker contacts. IEC56 (now IEC62271-100) specifies a minimum current of 50 A for this, while ANSI 37.09 specifies a minimum of 100 A. There are also many other applications, such as checking the continuity of rails in transport systems and busbars in switchboards, where a high continuous current is essential.

Portability. Carrying around a heavy test set, especially when working in hard-to-access locations, can be a real problem. Despite the need for high power operation, users wanted the new instrument to be compact, robust and to weigh as little as possible.

Use anywhere. The ability to operate from batteries was high on the wish list, as mains supplies are not always available in locations where low-resistance testing needs to be carried out. Of course, it must be possible to carry out a large number of tests without needing to recharge the batteries and, when the batteries are finally exhausted, the recharge time should be short. And operation direct from the mains should also be possible.

Noise immunity. Tests often have to be carried out in electrically noisy environments, such as substations. An instrument that gives unstable or unreliable readings in these circumstances can be more of a liability than an asset, especially when it’s necessary to compare or trend results over a period of time.

Safety. Users key requirement was a CAT IV 600 V rating in line with IEC 61010, not only for the test set itself, but also for the test leads and clamps.Safety is crucial and the users consulted were particularly concerned about effective protection against electric shock risks resulting from currents induced into the equipment under test by nearby live equipment. The best solution is support for DualGround™ operation, which allows the resistance measurements to be made with both sides of the equipment under test securely grounded. This is especially useful when working on circuit breaker contacts, where other test techniques require the ground on one side of the contact to be lifted before the measurement can be made.

Ingress protection. Testing isn’t always carried out in environments that are clean and dry. It was, therefore, considered highly desirable that the new instrument should have a high ingress protection rating – IP54, for example – and that this should apply not only with the lid closed for transport, but also when the instrument is in use with the lid open. An IP54 rating indicates excellent protection against the entry of dust into the instrument, and also protection against water splashes from any direction.

Smooth DC. An interesting point that emerged from the customer consultations is the importance of a smooth DC test current, with negligible ripple, especially when circuit breakers are to be tested. This is because several utilities have experienced problems with circuit breakers unexpectedly tripping while low resistance measurements were being carried out with instruments that had ripple in their DC output.

Usability. Customers involved in the consultation were emphatic that usability was a key issue. No matter how good the performance of an instrument, it will have little appeal if its features are made hard to access by a poorly designed user interface. There was a strong preference for a simple and intuitive interface based on clearly labelled rotary controls and a responsive but robust keypad.

Having listened to its customers, Megger has responded by launching the new DLRO100 high-current low-resistance test set. This very portable 100 A test set is the lightest of its type currently available. It can be powered from internal lithium ion batteries or from the mains, and it offers all of the other features asked for by customers and discussed in this article.
Unsurprisingly, the response to this new instrument has been enthusiastic, and it seems certain that it will soon be making life a lot easier for power engineers around the world, by letting them perform low-resistance tests just the way they want.