80 Years of Isometer – a Great Idea That Still Drives Us Today.

1937 Marks the Birth of the Isometer®.

Walther Bender was the one who had the groundbreaking idea. Whilst he was working as an inspection employee at TÜV Frankfurt/Oder in the 30s, one of his responsibilities was electrical safety in the pits of the Lower Lusatia lignite mine. With the technology at the time, the electrical installations had to be switched off in order to inspect them. This led to a great deal of resentment amongst the workers because they were paid per tonne, and whilst everything was switched off, they were not able to extract any lignite. On the other hand, the unearthed 500 V three-phase networks represented a huge safety risk requiring regular checks. As a thinker and a tinkerer, Walther Hans Bender could not get this dilemma out of his head. There had to be a way to inspect the installation during operation without causing any interruption! After much thought, and using all of his powers of innovation and ingenuity, he developed an “insulation monitoring and earth fault indication device for three-phase systems”.

In 1937, he thought the device was good enough to be submitted to the “Reichspatentamt” in Berlin. Today, the ISOMETER® is a trademark that is registered worldwide and is used successfully in a number of different applications besides mining.



Year of manufacture: 1965

These devices were used in unearthed three-phase networks up to 110 V. On the front, you can see the porcelain terminals that were common at the time. The devices were supplied in sheet steel housings or in completely enclosed insulated housings. At that time, glow lamps were used to indicate an insulation fault.


Year of manufacture: 1968

The A-ISOMETER® was designed for use in category 2 medical environments, such as operating theatres, ICUs, etc.


Year of manufacture: 1974

This device monitored alternate voltage auxiliary circuits of machines for insulation faults, and was used by several major automotive manufacturers in their
production lines.


Year of manufacture: 1975

The first more compact insulation monitoring device came on the market in the mid-1970s with the IRG100P. The constantly shrinking size of the parts allowed for new, more compact designs.


Year of manufacture: 1991

With the publication of DIN VDE 0107:1989, monitoring of the IT system transformer for load current and temperature became a requirement in addition to insulation monitoring. For the first time, an LC display was used for operation and display in the device.


Year of manufacture: 2003

With the increasing use of controlled drives, higher demands were placed on the measuring technology of insulation monitoring devices. These requirements have been met with the development of the patented AMP measuring method. Today, the A-ISOMETER® is world’s best-known standard device for these applications and is now used in networks with nominal voltages of up to 15,000 volts.


Year of manufacture: 2006

The current IR425 series is a state-of-the-art device for control circuits. Notifications via LCD display, electronics with SMD components and microcontrollers are what make up today’s “state-of-the-art”. But the heart of the equipment still comes from the minds of Bender’s development engineers – the software that determines the instrument’s measurement technology and functionality.


Year of manufacture: 2014

Even good old reliable technology has to adapt when the time is right.

With the iso685, Bender meets many of the requirements of industry 4.0. But this doesn’t just mean even faster measurement times, optimised history memories or flexible adaptation to different networks.

With iso685, Bender also provides its customers who depend on having reliable power supplies in their networks with a technically mature solution.