Siemens: Specifying Reliable DC Power Systems

DC Power Systems are the critical first step in designing an effective Automation system. We often overlook this crucial step, so our Automation & Control Experts share the considerations below to help guide you. Taken from our Tech Talks series, our goal is to help ensure that you have thought through all important factors in selecting the proper DC Power System.  To learn more about our Tech Talks series, click here.

Understand criticality
  • Health, safety, or environmental impacts?
  • Economic Consequence of failure?

a. Hazardous Area

  • None = general purpose
  • NEC 500 Class 1
  • ATEX Zone 2
  • IEC Ex Zone 2
  • Other

b. Enclosure Requirements

  • IP
  • NEMA
  • Maximum temperature
  • Minimum temperature
  • Space constraints (height, width, and depth)
Control system integration requirements
  • Remote accessible for troubleshooting and configuration?
  • Siemens TIA compatible (Leveraging PLC function blocks and HMI faceplates for system for status, diagnosis, control, and troubleshooting.)
Power system
  • Output Voltage:
  • Output Current:
  • Input voltage range and frequency (e.g. 85V to 550V AC at 50/60 Hz)
  • Efficiency requirement? (e.g. >= 90% efficiency)
  • Power boost requirements?
  • Relay fault contact?
  • Adjustable output voltage?
Multiple independent loads
A selectively module protects, isolates, indicates, and controls 4 independent circuits. The selectivity module keeps a fault on one circuit from propagating on the other circuits. Without isolation a single short or overload could trip out the power supply and impact independent control functions.
  • Number of independent isolated loads/feeds?
  • Load per circuit?
  • Local indication needed?
  • Remote indication per circuit trip and remote reset capable?
  • Control needed – ability to de-energize and re-energize each circuit?
Siemens Selectivity Module Highlights:
Each of the 4 power circuits can have an adjustable potentiometer to set trip points up to 10 amps per circuit. The selectivity module also allows on/off control of each circuit with the push of a button and 3 color LED indication per circuit to identify the circuit’s current state (on/off/tripped). Another cool feature of the selectivity module is the ability to send out a failed circuit status to the control system and have the control system remote reset once the problem has been addressed. This solution is less expensive than independent power supplies per circuit, and provides fast diagnosis resulting in less downtime.
DC backup during source power outages
A UPS can maintain defined power levels (buffer) for a specific period after AC power is lost, or there is a DC power supply failure. Enabling safety shutdown sequences, closing out a batch of product, or tracking and logging critical data after a power loss.
a. Amount of time to buffer:
  • # of seconds, minutes, hours, days
b. Backup load
  • Percent of normal load
  • Plus UPS charging load
c. Space constraints
  • Height
  • Width
  • Depth
d. Lower power and buffering times
  • Capacitor-based UPS
e. Siemens UPS Considerations:
  • When the application fits, capacitor-based systems should be selected over batteries. Capacitors last significantly longer than batteries, they have a wider temperature range, they charge faster, they do not emit a hazardous gas and therefore don’t require encloser venting. There is less maintenance on a capacitor-based system over a battery-based DC buffering system. If your system requires less that 15 amps of DC buffering for a buffering time measured in seconds to minutes, the UPS500S is a great solution. For applications needing up to 40 amps of DC buffering time measured in minutes to hours, the UPS1600 with external battery modules is the right solution.

  • The Siemens capacitive based UPS500S has a 15-amp total load capacity. Buffer time at 15 amps is 9 seconds to 40 seconds. Buffering time with a 1-amp load is 138 seconds to 572 seconds. Additional buffering modules are required to achieve greater DC buffering times. Refer to KT10.1 SITOP catalog ( for the buffering time chart bases on load and the number of buffering modules.
Redundancy (High Consequence of Failure)
When the consequence of failure is high, DC power system redundancy should be considered. DC bus failure is often caused by the unintentional flipping of the up-stream breaker that feeding the power supply, or overloading, or shorting out the DC side of the power supply. Adding a second DC power supply and redundancy module can address both situations and allowing the DC bus to remain powered.
  • Quantity of redundant power supplies
  • Load balancing required
  • Excess of 20 amps of redundancy required
  • Yes – 2 redundancy modules are required.
  • No single point of failure required
  • Yes – 2 redundancy modules are required.

a. SIL 3 (i.e. HIPPS)

  • System needs to withstand 3 faults while maintaining power to the DC bus and supply DC power to the critical output devices.
Siemens Redundancy Considerations:
The Siemens redundancy module monitors the primary input and will switch to the secondary input upon DC power loss. This module also provides the diode protection that will keep the output of each power supply from inducing power on the other power supply. There is a status contact relay that can tell the control system when a power supply is no longer functioning. Please note that the redundancy module is rated for 40 amps. This means only two 20-amp power supplies can be utilized providing 20 amp of redundancy using a single redundancy module scheme. If 40 amps of DC redundancy is needed, then using two redundancy modules and two 40-amp power supplies are required.
Why do we love the Siemens modular DC power architecture?
  1. Wide range AC input supply voltage of 85V to 550V AC. The widest range on the market that will work in all areas of the world and require less models to
    carry, supply, and support.
  2. Wide temperature range of -25 deg C to +70 deg C and an extremely slim design with no lateral installation clearance requirements. Can use these DC
    power systems in the heat of Saudi Arabia and the cold of Northern Canada without additional panel heating or cooling. Can place these power supplies
    side by side on a standard din rail saving rail space for more items.
  3. Triple hazardous area rating of UL Class 1 Division 2, ATEX zone 2, and IEC Ex zone 2. Can place all over the world with less models to carry,
    supply, and support.
  4. Adjustable potentiometer for DC voltage adjustment of 24V to 28.8V. Voltage adjustment to offset voltage drops on long cable runs or when multiple
    devices are powered off the same circuit.
  5. Power boost feature allows 3 X the rated current (for 25ms) for tripping protective devices downstream of the power supply. Fast overload conditions do
    not trip the power supplies internal circuit breaker causing loss of the DC power before tripping the intended circuit breaker or fuse downstream. 1.5
    times rated current (5 s/min) to address starting inrush currents.
  6. Normally open “DC OK” relay contact block which can be tied back to the control system for remote DC power indication.
  7. The Modular series is 94% efficient, the highest in the industry reduces energy loss and heat impacts.
  8. 5A, 10A, 20A, and 40A models work in 50Hz and 60HZ systems, single phase, dual phase, and three phase AC supply applications.
  9. AWC extensive stock and competitive pricing.
Tracy Spafford is VP of Automation Technology for AWC Inc. He is a control systems specialist and consultant with over 31 years of experience in industrial automation, and he has a BS in Electrical Engineering Technology from Texas A&M University.

Tracy Spafford

V.P. of Automation

Tracy Spafford

V.P. of Automation

Tracy Spafford is VP of Automation Technology for AWC Inc. He is a control systems specialist and consultant with over 31 years of experience in industrial automation, and he has a BS in Electrical Engineering Technology from Texas A&M University.
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