Murrelektronik and the disenchanting of the loop resistance

What on earth is a "channel monitor"! And then also in connection with the term "selective"? Images of gigantic ocean-going vessels quickly form on their way through the Kiel Canal, saving themselves the 250-mile detour through the North Sea, Skagerrak and Kattegat. And because the pilot prefers to check his message status on his smartphone instead of doing his actual work, he only monitors the channel passage selectively!

If circuit breakers do not work

While this is a nice interpretation of "selective channel monitoring", it is simply wrong. Experts who have spent hours troubleshooting a machine know this. This is particularly costly in complex systems in which switched power supplies electronically regulate the voltage and current at the output.

It is possible that in the event of a short circuit or overload, secondary fuses react more slowly than the power supply unit and therefore this selectivity is not applicable. This leads to critical situations, such as voltage dips, and in the worst case even to cable fires. But how is it possible that these downstream protective devices do not react? This requires a look back almost 30 years.

Convincing argument: high short-circuit protection

It was at the beginning of the 1990s when a change in the mechanical and plant engineering industry was about to take place: the change from transformers to electronic power supplies. At the beginning, only a small group of people dared to take advantage of the new devices. A regulated 24 V DC voltage and short-circuit protection according to a fixed defined characteristic curve obviously sounded too good to be true for potential users!

However, the rise of electronically regulated power supplies was unstoppable from then on, as more and more OEMs wanted to benefit from their advantages. Above all, the high short-circuit protection was a convincing argument. If a short-circuit went unnoticed in the transformer power supply units used up to then, it heated up the subsequent installation and possibly even set fire to it. With the electronically regulated power supplies, on the other hand, users were buying modern technology and at the same time greater operational reliability.

The search for short circuits outside the control cabinet

But how was it with short circuits outside the switch cabinet? Output-side miniature circuit breakers, in practice often combined with a signal contact going to the control system, reliably detected overloads and short circuits in the field. So why should this form of protection, which has proven itself over decades, not be retained? What was good and right for a transformer power supply unit, according to many users, had to be even better for an electronically regulated power supply! This false assumption caused many an electrician to despair in troubleshooting in the years to come. For example, if the reason for this fault was a bare cable in a drag chain, simply isolating the fault could take many hours, if not several days.

Loop resistance as an evil

But how could it be that the switch-mode power supplies with their advantages were not capable of reliably tripping miniature circuit breakers? This question not only drove the manufacturers of electronically regulated power supplies up in arms, but also drove suppliers of automation solutions to experiment.

It is no longer possible today to determine who was allowed to claim the exclamation "eureka" in the end. However, that is not so important either. Much more interesting is the result of countless tests and calculations - especially since these revealed a banal reason for the phenomenon of non-tripping miniature circuit breakers: loop resistance! The electronically regulated power supplies so enthusiastically celebrated by the market were simply not able to provide the current required for tripping for at least 100 ms because of this.

Video: Mico Pro – current monitoring modularized

The calculation of the loop resistance

So, the loop resistance! In order to understand why this latest technology, of all things, is problematic, it is necessary to take a detour into the basics of mechanical and plant engineering. Up until 30 years ago, it was actually common practice to use type C miniature circuit breakers to protect installations in the field. What this means in combination with a switch-mode power supply is explained by an example in which an automatic machine with 6 A rated current is used. According to the formula 14 x Inenn, this requires a tripping current of 14 x 6 A, which in multiplication corresponds to 84 A. However, for a 24 V power supply to be able to provide this 84 A at all, its resistance must not exceed 286 mΩ.

That this resistance value is unrealistic is shown by a practical example, where the loop resistance of a 5 m long sensor cable with a wire cross-section of 0.34 mm2 is calculated. Its resistance is calculated from the formula R = ρ x l / A, where l is multiplied by two due to the outgoing and return lines.

If the individual values are now used taking into account the resistivity ρ of copper (0.0178 Ω x mm2/m), the result is already a resistance of 520 mΩ. With the additional resistances of the distribution line and the strands as well as the internal resistances of the miniature circuit breakers and terminals, the total resistance adds up to over 1.3 Ω.

Applied to the formula U = R x I, this means that a maximum current flow of 18.18 A is possible in an electronically regulated 24 V power supply. However, this is not sufficient to trip a type C circuit breaker with a rated current of 6 A. As described, it would require at least 84 A.

Explanations on the functioning of miniature circuit breakers and protective resistors. Watch the video:

Video: Video Functionality of miniature circuit breakers and protective resistor