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LED Status Indicators
Use in Fuse Terminals and Modules
Fuse holders used in industrial applications, whether fuse terminal blocks
or modules, often include a status LED. These may indicate the fuse is healthy
or may energize if the fuse fails and thus indicate a blown fuse.
Fuse healthy indication requires a neutral or 0V connection. The LED
circuit is located between the load side of the fuse and neutral or 0V. Often a
green LED is used but red LED’s are also common.
Figure 1 - Fuse Healthy Indication
In control systems, if the terminal is located between the power supply and
the digital I/O the indicator will show the fuse status regardless of the state of the
I/O. For example, a fuse may be located between the power supply and a
transistor output on a PLC. The output does not need to be energized for the
fuse healthy indicator to operate.
If the fuse terminal is located between the digital output and the field
device or load then the output must be energized for the status indication circuit
to operate.

Figure 2 - Connection to PLC Digital Output
When the fuse fails the indicator turns off so there is no indicator current
produced in a blown fuse situation. Fuse modules, i.e. a printed circuit board in a
DIN rail mounted housing, can include the neutral / 0V connection so fuse
healthy indication is easily achieved. However, most fuse terminals do not
include terminations for the neutral / 0V so this type of status indication is less
common than blown fuse indication.
In the case of blown fuse indication, the LED circuit is located in parallel
with the fuse. When the fuse is healthy the voltage drop across the indicator
circuit is insufficient to energize the LED. If the fuse blows the supply voltage is
now applied to the indicator circuit and the LED illuminates. As the impedance of
the LED circuit is generally much higher than the load the majority of the supply
voltage is applied to the LED circuit.
Figure 3 - Blown Fuse Indication
Figure 4 - Connection to PLC Digital Output

No terminations for the neutral / 0V are required. The drawback is the
LED circuit produces a current flow through the load when the fuse is blown.
This “leakage current” is undesirable and should be kept to a minimum so that it
does not pose a safety hazard. LED currents as high as 2.5-3mA are not
uncommon. At least one DIN rail fuse terminal supplier offers blown fuse
indication that draws 0.25-0.5mA and still provides highly visible LED’s.
Another drawback with blown fuse indication is that the indicator only
functions when the PLC output is energized.
Figure 5 - Leakage Current
Emphatec’s new DEPRO - CLSI offers the best of both worlds - no
leakage current fuse healthy indication with no need to provide a neutral / 0V
connection. This is achieved by installing current sensing in series with the fuse.
If the fuse is powered and healthy and the load is connected (and not open
circuit) then the indicating LED energizes. Like blown fuse indication the
indicator circuit only works when the PLC output is energized.
Figure 6 - DEPRO - CLSI Fuse Healthy Indication
The current sensing circuit must be compact (current transformers would
be too large) and introduce minimal voltage drop. Emphatec’s circuit drops the
voltage by approximately 0.8V regardless of the current flow so to keep power

dissipation to a minimum the maximum fuse rating is 500mA. This is more than
sufficient for the majority of PLC I/O and control loop applications.
The DEPRO - CLSI is designed for both AC and DC voltages and
operates from 12V to 230V - one fuse module will work in a wide range of
applications. The LED intensity is not dependant on the supply voltage but is
optimal at a load current of 10mA of higher. At lower load currents it may be
difficult to see the LED when illuminated.
Key Features of the DEPRO - CLSI:
- Wide operating voltage range: 12-230Vac/dc
- Fuse healthy indication without the need for a neutral or 0V connection
- No leakage current
- Indicates control loop wiring is intact, fuse is healthy and load is drawing current
- Requires just 11.2mm of DIN rail
- Mounts on 32mm and 35mm DIN rails
- Class 1 Division 2 certified for use in hazardous locations