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Voltage Drop Consideration
With the potential of using up to 32 taps and long cable lengths between taps,
the voltage available to the safety devices at connector J3 must be calculated.
The voltage available to the safety device has two components:
• The voltage drop due to the wire resistance of the cables
• The voltage drop within the tap
The resistance of the recommended 18 AWG cordsets and patchcords is
(0.0218 ohms/m (0.00664 ohms/ft). The wire resistance of the cordset from
the power supply to tap 1 (R
R
= 0.0218 * L
1
The wire resistance must be considered for both the power and ground;
therefore the voltage drop is multiplied by two. The voltage at connector J1 of
tap T1 (V
) is:
J1
V
= 2 * I
* R
J1
1
1
The tap has a small voltage from connector J1 to J2. The typical voltage at
connector J2 (V
) drop through the tap from J1 to J2 is:
J2
V
= V
- (2 * 0.028V)
J2
J1
The voltage available at connector J3 is dependent on the device that is
connected to J3. The typical voltage drop from J1 to J3 is 0.4V when the device
uses 50 mA.
V
= V
- 0.4V (typical)
J3
J1
IMPORTANT The voltage drop from J1 to J3 can be as high as 1.2V with a maximum load
of 500 mA at the highest rated ambient temperature.
The taps consume 25 mA when OFF. The EMSS taps consume an additional
15 mA (7.5 mA per channel) when the contacts are closed. The OSSD taps
consume an additional 6 mA (3 mA per channel), when the outputs are ON.
A spreadsheet can be used to calculate the voltage available to the safety device.
Table 4 on page 26
shows the voltage available to the safety device of a number
of different devices. Assuming that the power supply voltage is set to 24V, and
the cable is the recommended 18 AWG, the voltage available to the safety
devices is shown in the right-hand column.
When guard locking devices are used in the circuit, the taps and wiring
components are subjected to momentary surges in current. With the sequential
operation of the lock/unlock command, the momentary surges should not
adversely affect the performance of the GuardLink circuit.
Rockwell Automation Publication 440R-UM015C-EN-P - April 2018
) is:
1
1
GuardLink System Design
Chapter 2
25
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