Back to overview


Overload currents and short-circuit currents are usually unexpected. They cause malfunctions and interruptions to the ongoing operation of a system. Production downtimes and repair costs can often be the unfortunate consequences.

Minimize damage by protecting individual devices or device groups separately. In this way, termination devices are optimally protected against damage or destruction. System parts, which are not in the affected circuit, continue to operate without interruption, insofar as the overall process allows.

Nominal load currents

Electrical loads  

Electrical loads with various nominal currents

In the case of various nominal currents, we recommend protecting the circuits separately. Suitable device circuit breakers are available for every nominal current.

Here are some examples:

  • Valves: 0.5 to 4 A
  • Motors: 1 to 12 A
  • Relays: 0.5 to 5 A
  • Controllers: 1 to 8 A
  • Sensors: 0.5 to 2 A
Back to top

Overload currents

Motor with overload current display  

Shutdown of overload currents in the seconds to minute range

Overload currents occur if termination devices unexpectedly require a higher current than the rated current provided. Such situations may arise, for example, due to a blocked drive. Temporary starting currents from machines are also considered to be overload currents. The occurrence of these can essentially be calculated, but nonetheless can vary depending upon the machine load at the moment it starts.

When selecting suitable fuses or circuit breakers for such circuits, these conditions should be taken into account. Safe shutdown should occur in the seconds to minute range..

Back to top

Short-circuit currents

Motor with short-circuit current display  

Shutdown of short-circuit currents in the milliseconds range

Short circuits may occur in the case of damage to the insulation between conductors, which carry operating voltage. Typical protective devices for shutting down short-circuit currents include fuses or miniature circuit breakers with various tripping mechanisms.

Short-circuit currents should be reliably shut down in the milliseconds range.

Back to top

Residual currents

Residual currents occur when insulation becomes damaged and in the event of short circuits between live parts and the ground. Such errors can result in life threatening contact voltages for humans and animals.

Residual current devices switch off system parts in which such errors occur within a few milliseconds. Such protective devices are not taken into consideration here.

Back to top

Influence of cable lengths on shutdown behavior

In the event of an error, long cable paths limit the required tripping current. They can delay or even prevent switch-off.

The maximum cable lengths that can be used between a power supply unit and a termination device are defined by the following criteria:

  • The power supply's maximum current
  • Internal resistance of the circuit breaker
  • Cable resistance

The cable resistance is dependent on the cable length and conductor cross section. For this reason, as a general rule, the shortest cable path should be selected during installation.

Power supply and load

The length and cross section determine the switch-off conditions for a device circuit breaker

Cable resistance counteracts a short-circuit current. In the event of low voltage sources, a short-circuit current can be limited by the cable resistance in such a way that safety equipment no longer recognizes this current as a short-circuit current. In the case of circuit breakers with C characteristics, for example, the upper tripping limit is significantly higher than the nominal current. For this reason, a delayed switch-off is highly likely in the event of a short circuit when using this safety equipment.

Optimized protective devices with SFB characteristics or active current limitation detect at an early stage whether the nominal current has been exceeded.

Back to top

Cable calculation

To calculate the maximum useable cable length, the following data is required:

RmaxMaximum total resistance
UNominal voltage
ICBRated current of the device circuit breaker
xITripping factor according to current characteristic curve/multiple of the nominal current
RLmaxMaximum cable resistance
RCB1AInternal resistance of the device circuit breaker 1A
LmaxMaximum cable length
AConductor cross section
ρSpecific cable resistance Rho, (Cu 0.01786)
Back to top


These values form the basis of the following calculation example:

U24 V DC
xI15 (from M1 characteristic curve)
RCB1A1.1 (from the table for nominal currents and internal resistances of thermomagnetic circuit breakers)
P0.01786 (copper)
A1.5 mm2 (assumed)
Back to top


Example calculation for cable length  

Example calculation for cable length

The calculation example can be viewed here in three steps:

  1. Total circuit resistance
  2. Maximum cable resistance
  3. Maximum cable length
Back to top

Main and auxiliary contacts

Main and auxiliary contacts of device circuit breakers  

Setting the auxiliary contacts based on the switching state of the main contact

A number of device circuit breakers have additional auxiliary contacts. They enable remote querying of switching states and malfunction notifications.


Power = main contact
Signal = auxiliary contacts
NO = normally open
NC = normally closed
C = common PDT foot contact

Back to top

Marking of connections

Main contactsIndividually: 1 - 2
 In groups: 1 - 2/3 - 4/5 - 6/...
Auxiliary contactsIndividual N/O contact: 13 - 14
 N/O contacts in groups: 1.13 - 1.14/2.13 - 2.14/3.13 - 3.14/...
 Individual N/C contact: 11-12
 N/C contacts in groups: 1.11 - 1.12/2.11 - 2.12/3.11 - 3.12/...


C6 The Exchange
Calmount Park
Dublin 12
D12 XE18