KNX Buffer Modules: Uninterruptible Power for Safety-Critical Functions
Safety-critical KNX functions — fire shutter actuators, emergency HVAC, stairwell lighting relays, and access control — cannot tolerate the 5–30 second bus restart that follows a mains failure. Buffer modules eliminate this gap by maintaining 29V bus power from a supercapacitor array during mains interruptions.
Why buffer the KNX bus?
When mains power fails, an unbuffered KNX PS640 loses output within milliseconds. All bus devices power down. On mains restoration, each device performs an initialisation sequence: reading stored parameters from EEPROM, sending programming mode queries, and joining the logical line. This restart process takes between 5 and 30 seconds depending on device complexity.
For most KNX functions — lighting, blind control, temperature setpoints — a brief restart is acceptable. For safety-critical outputs, it is not:
Unacceptable restart scenarios
- Fire shutter actuator restarts during a fire event — shutter may be stuck mid-position
- Access control door lock relay releases during a power blip — security breach
- Smoke extract damper actuator restarts — unknown blade position for 15 seconds
- Emergency HVAC relay drops during an event — loss of pressurisation control
KNX functions that tolerate restart
- Standard lighting actuators — brief off then resume last state
- HVAC zone valve actuators — off during restart is acceptable
- Blind and shutter actuators in non-safety contexts
- Metering and energy monitoring devices
Buffer module operating principle
A KNX buffer module contains a bank of high-capacity supercapacitors (electric double-layer capacitors, EDLC). During normal mains operation, these capacitors are charged from the KNX bus 29V supply. On detection of mains loss, a DC/DC converter switches to discharge mode, drawing energy from the capacitor bank and maintaining the bus voltage at 29V ± 0.5V. KNX devices connected to the bus detect no interruption in supply — they continue operating normally throughout the mains failure.
Buffer module energy storage principle
Normal operation:
Mains → PS640 → 29V bus → devices (operating)
↓
Buffer module charges supercapacitors
Mains failure detected (within 10ms):
PS640 output drops
Buffer module DC/DC converter activates
Supercapacitors → DC/DC → 29V bus maintained
Devices: no interruption, no restart
Backup duration depends on:
- Supercapacitor capacity (Wh)
- Total bus current draw (mA)
- DC/DC converter efficiency (~85%)
Backup time ≈ (Capacitor Wh × efficiency) / (Bus W)
Bus W = 29V × I_total (A)Supercapacitors vs batteries: supercapacitors charge and discharge in seconds to minutes, support hundreds of thousands of charge cycles, and do not degrade significantly over a typical building lifetime (20+ years). Lithium batteries offer higher energy density for longer backup times but require replacement every 3–5 years. Standard DIN-rail buffer modules use supercapacitors; external battery buffer modules add lithium cells for multi-hour backup.
Gira 2093 buffer module
The Gira 2093 is the most widely specified standalone KNX buffer module in European commercial installations. It occupies 2 DIN-rail modules, contains a 1.5 Wh supercapacitor bank, and is connected in parallel with the PS640 output.
| Line loading | Bus current draw | Approximate backup time |
|---|---|---|
| Light load (~10 devices) | ~80 mA | ~2 hours |
| Medium load (~25 devices) | ~180 mA | ~45 minutes |
| Heavy load (~40 devices) | ~300 mA | ~25 minutes |
| Near-maximum (~60 devices) | ~500 mA | ~15 minutes |
Gira 2093 wiring — parallel connection to PS640
PS640 output terminals: + (red) → KNX bus + rail - (blue) → KNX bus - rail Gira 2093 connections: + terminal → same KNX bus + rail (parallel with PS640) - terminal → same KNX bus - rail (parallel with PS640) Polarity is critical — reverse connection damages the module. Connect before powering up. LED indicator: Green solid = charging from mains (normal) Green blink = fully charged, mains OK Amber/red = on buffer power (mains lost)
MDT PS640 with integrated buffer
MDT offers the SCN-PS064.01 power supply with an integrated buffer capacitor bank option. This combined unit provides PS640 functionality and buffer capability in a single 4-module DIN-rail enclosure, saving panel space versus a separate PS640 plus standalone buffer module (which would require 4 + 2 = 6 modules total).
Advantages of integrated unit
- Saves 2 DIN-rail modules versus separate buffer
- Single connection point — no external parallel wiring needed
- Current monitoring via KNX status object (bus current in mA)
- Buffer status telegram available as KNX group address (DPT 1.001)
When to use separate buffer
- Existing PS640 already installed — add Gira 2093 without replacing PSU
- PS1280 required — use standalone buffer in parallel
- Different PS manufacturer specified — add Gira 2093 universally
Calculating required backup time
The required backup time depends on the specific safety standard governing each safety-critical KNX function. Use the requirements below to specify the correct buffer module or lithium battery system:
| KNX function | Standard | Required backup time | Buffer module selection |
|---|---|---|---|
| Fire shutter actuators | EN 15650 / building regulation | 30 min (stroke) + 30 min monitoring = 1h | Gira 2093 adequate for light-load lines |
| KNX access control door locks | BREEAM / EN 50133 recommendation | 4h minimum | Lithium battery module required |
| Emergency HVAC dampers | Project-specific / FM | 1–2h | Lithium battery module recommended |
| Stairwell emergency lighting relay | EN 50172 | Battery ECG on separate circuit — KNX backup optional | Supercapacitor buffer sufficient if KNX backup desired |
| Smoke extract VAV dampers | EN 12101-6 | 1h minimum (test and monitoring) | Gira 2093 or small lithium module |
Lithium battery buffer modules for extended backup
For backup requirements of 1–8 hours, supercapacitor buffer modules are insufficient. Lithium battery buffer modules such as the MDT SCN-USB55S.01 or ABB SU/S series connect an external 12V lithium battery to the buffer module, providing extended backup through a DC/DC converter that maintains the 29V bus voltage.
Battery capacity calculation — extended backup
Formula:
Battery capacity (Ah) = (I_bus × V_bus × t_hours) / (V_battery × η)
Where:
I_bus = total bus current (A)
V_bus = KNX bus voltage (29V)
t_hours = required backup time (h)
V_battery = external battery voltage (12V)
η = DC/DC converter efficiency (~0.85)
Example: 40-device line, 200mA bus current, 8h backup required
Capacity = (0.200 × 29 × 8) / (12 × 0.85)
= 46.4 / 10.2
= 4.55 Ah → specify 6Ah or 12Ah for margin
Battery replacement interval: typically 3–5 years
(check manufacturer specification; lithium degrades with cycles)
Include replacement schedule in maintenance documentation.Battery buffer is NOT additional current capacity: connecting a buffer module (supercapacitor or lithium) in parallel with a PS640 does not increase the available bus current during normal operation. The buffer only provides backup power when the PS640 output drops. Current capacity during normal operation remains limited by the PS640 (640 mA).
Panel installation and wiring
Buffer modules must be installed adjacent to the PS640 on the same DIN rail and connected directly to the KNX bus bars. Follow these rules for correct installation:
Panel installation checklist
1. Mount buffer module immediately adjacent to PS640 on DIN rail
2. Connect + terminal to KNX bus + (red bus bar)
3. Connect - terminal to KNX bus - (black/blue bus bar)
4. Verify polarity before energising — reverse polarity destroys module
5. Label buffer module with:
- Manufacturer and model number
- Backup time at current load (from table or calculation)
- Date of installation (supercapacitor models) or
battery date + next replacement date (lithium models)
6. Record buffer module in panel schedule
7. Test backup function during commissioning:
- Disconnect mains to PS640
- Verify bus remains powered (devices still active)
- Reconnect mains — verify smooth transition backNeed a KNX panel with buffer modules specified and installed correctly?
We design KNX panels with backup time calculations per safety standard, correctly specified buffer modules, lithium battery sizing, and commissioning test documentation — delivered tested and certified to your site.
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