UPS Integration in KNX Panels: Battery Backup and PELV Compliance
KNX buffer modules handle minutes of backup on the bus alone. When an entire KNX panel — including 230V actuator supplies, IP routers, and load-side outputs — must remain operational for hours during a mains failure, a panel-level UPS is required. Correct sizing, topology selection, and PELV isolation are critical for safe and compliant integration.
When a panel-level UPS is required
A KNX bus buffer module protects only the 29V KNX bus and devices powered from it. It does not protect the 230V AC supply rail that powers actuator switched-mode power supplies, KNX IP router brick adapters, or 24V DC auxiliary power supplies for access control and security panels. When the entire panel must survive mains loss, a UPS upstream of the panel input MCB is required.
Applications requiring panel UPS
- Hospitals and healthcare facilities — HTM 06-01 compliance
- Security control rooms — BS EN 50131-6 Grade 3/4
- Data centres — UPS covers KNX access control panels
- Hotels and hospitality — guest room panels with DND / make-up room functions requiring always-on status
- Critical HVAC control panels where shutdown is unacceptable
Buffer module suffices when
- Only KNX bus continuity required (no 230V loads during backup)
- Backup time under 2 hours for light bus loads
- Fire shutter stroke and monitoring period only (~1h)
- Budget constrained — buffer module is 10–20× cheaper than UPS
UPS topology selection
UPS systems are classified by topology, which determines transfer time (the gap between mains failure and battery output) and output quality. For KNX panels, topology choice depends on the application and the sensitivity of connected equipment.
| Topology | Transfer time | KNX impact | Recommended for |
|---|---|---|---|
| Offline / standby | 4–10 ms | PS640 internal capacitor maintains 29V during 10ms gap — acceptable for standard KNX | Standard KNX panels in commercial buildings |
| Line-interactive | 2–4 ms | Acceptable — AVR corrects voltage sags without switching to battery | KNX panels with some sensitive loads; more efficient than online |
| Online double-conversion | 0 ms (zero transfer) | Recommended for hospital, security and data centre KNX panels | Critical KNX installations; devices never see any supply interruption |
UPS models for KNX panel integration
The two most common UPS form factors for KNX panel integration are tower/rack UPS placed beside or near the panel enclosure, and DIN-rail UPS modules mounted inside the panel. DIN-rail UPS are limited to approximately 1000 VA but eliminate external UPS enclosure requirements.
APC Smart-UPS SC 650VA (tower)
- 650 VA / 400 W output capacity
- Input: 230V 50Hz; output: 230V AC (pure sine online)
- 4 × IEC C13 output sockets
- USB and RS-232 communication ports
- Battery runtime at 100W load: approximately 8 minutes
- Replace battery when indicator shows 40%+ capacity decline
- Optional SNMP card AP9630 for network monitoring
DIN-rail UPS options
- PowerWalker VI 1000 RLE: 1000 VA, DIN-rail, 230V output, USB port
- Salicru SPS Advance: wall-mount, 1000 VA, dry contact output (NO/NC)
- Eaton 5S IEC: compact DIN-mount version available for panel integration
- Runtime at 75W load: approximately 15–20 minutes (varies by battery size)
- DIN-rail UPS typically consume 8–12 DIN modules inside the panel
Battery replacement planning: UPS battery life is 3–5 years under normal conditions (20°C ambient, regular charge cycles). In warm panel environments (above 25°C), battery life decreases significantly — plan 2–3 year replacement intervals in warm enclosures. Include battery replacement in the annual maintenance contract for all KNX panels with UPS.
Panel load calculation
Before selecting UPS capacity, calculate the total panel load that the UPS must support. Include every device powered from the UPS output — not just the KNX bus supply.
Panel load calculation — example KNX commercial panel
Component Typical power draw ----------------------------------------------------------- KNX PS640 (29V/640mA bus supply) 20 W KNX IP router brick power supply 5 W 24V DC SMPS for actuator auxiliary 30 W Access control 12V PSU (0.5A × 12V) 6 W KNX IP gateway (LAN side) 5 W Panel internal lighting and socket 5 W ----------------------------------------------------------- Total panel load 71 W UPS selection: Load = 71 W 650 VA UPS at 0.6 power factor = 390 W capacity ✓ Battery runtime at 71W load (650VA APC SC): Approximately 25–30 minutes (consult manufacturer runtime chart) If runtime < required: step up to 1000VA or add external battery pack
Do not undersize the UPS: operating a UPS above 80% of its rated watts reduces battery runtime and shortens battery life. If the calculated load is above 80% of UPS rated watts, size up to the next UPS capacity. A common mistake is specifying UPS VA without converting to watts using the power factor of the load.
UPS monitoring via KNX group addresses
Integrating UPS status into KNX enables automatic building responses to mains failure: switching off non-critical loads, arming security, and sending push notifications via Home Assistant or Gira X1. Two integration methods are available depending on UPS capabilities.
Method 1: APC Smart-UPS with SNMP card (AP9630)
APC Smart-UPS + AP9630 SNMP card
→ Network (LAN) → Home Assistant (apc_ups integration)
→ Home Assistant KNX integration
→ KNX group address write: DPT 1.001
0 = on mains power (normal)
1 = on battery power (mains failed)
ETS6 reaction to on_battery = 1:
→ Activate "power failure" scene:
- Non-critical lighting circuits: OFF
- Security system: ARMED
- HVAC: minimum ventilation mode
- Access control: locked down or free exit (project-specific)
→ Gira X1 / Homeserver: push notification to facility managerMethod 2: Dry contact to KNX binary input (Salicru / any basic UPS)
Basic UPS with NO/NC dry contact relay output:
Relay closes on battery power (or relay opens — check datasheet)
Wiring:
UPS dry contact COM → MDT BE-0800.01 input terminal (e.g. IN1)
UPS dry contact NO → 12V auxiliary supply (MDT BE-0800.01 requires
12–24V AC/DC at input)
ETS6 programming:
BE-0800.01 IN1 object (DPT 1.001) → send on change → GA "UPS status"
GA "UPS status" = 1 → trigger power failure scene (same as SNMP method)
Cost: ~€80 for MDT BE-0800.01 vs ~€200 for SNMP card
Limitation: no runtime remaining or battery health data in KNXPELV and SELV isolation requirements
KNX TP bus operates at 29V DC SELV (Safety Extra Low Voltage). The UPS output is 230V AC mains. IEC 60364-4-41 requires that SELV circuits are physically and electrically separated from mains circuits to maintain their safety classification. Failure to maintain this separation risks the KNX bus being energised at mains potential during a fault — a life-threatening hazard.
IEC 60364-4-41 SELV separation requirements
KNX bus = SELV circuit (29V DC, transformer isolated)
UPS output = 230V AC MAINS circuit
Requirements (IEC 60364-4-41, clause 414):
1. SELV conductors must be physically separated from mains conductors
Minimum: 50mm physical spacing in open cable tray, OR
Insulating barrier (PVC or polycarbonate divider in duct), OR
Individual conduit for SELV cables separate from mains conduit
2. SELV conductors sharing cable duct with mains:
Each SELV conductor must be insulated for 230V AC
(use 450/750V insulated wire even for SELV circuits)
3. 4kV impulse withstand between SELV and mains circuits
(maintained by KNX PS640 internal isolation transformer)
4. SELV connectors must not be intermateable with mains
(KNX TP uses screw terminals — segregation is physical only)
In KNX panel layout:
- Route KNX bus wires (green/red) in separate cable duct from
230V UPS output wiring (brown/blue/green-yellow mains cables)
- Minimum 50mm between KNX bus cable duct and mains cable duct
- Add insulating barrier strip between KNX section and 230V section
of DIN-rail if both are in the same panel enclosurePELV distinction: PELV (Protective Extra Low Voltage) differs from SELV in that PELV circuits may be earthed. KNX bus is SELV — it must NOT be earthed (no connection between the KNX bus conductors and PE). If KNX bus screen is earthed at the PS640 end (common practice for EMC), this is acceptable as the screen is not a current-carrying conductor — the bus conductors themselves remain isolated from earth.
Documentation and maintenance
Every KNX panel with UPS integration requires specific documentation for maintenance compliance and insurance purposes. Record the following in the panel schedule and O&M manual:
UPS documentation requirements
Panel schedule — UPS record:
UPS manufacturer and model: APC Smart-UPS SC 650VA
UPS topology: line-interactive
Rated capacity: 650 VA / 400 W
Measured panel load at commissioning: 71 W
Battery runtime at commissioning load: 28 minutes
Battery model: APC RBC17
Battery installation date: [date]
Next battery replacement due: [date + 4 years]
Annual maintenance inspection:
1. Check UPS front panel for any fault/battery LEDs
2. Initiate self-test (via front panel or PowerChute software)
3. Record battery runtime from self-test result
4. Compare to commissioning runtime — replace battery if < 50%
5. Check that UPS KNX integration is functional:
Switch UPS to battery manually → verify KNX power failure scene triggers
6. Check UPS input and output wiring for signs of overheatingNeed a KNX panel with UPS integration and PELV compliance documented?
We design KNX panels with UPS load calculations, correct SELV separation layout, KNX group address monitoring integration, and full O&M documentation — delivered tested and certified to your site.
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