Impulse relays or a bus-based system for office lighting? For most cellular and small open-plan offices, DIN-rail impulse relays switched from local push-buttons remain the simpler and cheaper answer — one relay per lighting zone, no addressing, no commissioning software. A bus-based system (KNX, DALI or a proprietary lighting-management bus) earns its cost when the office needs per-luminaire dimming, daylight harvesting, occupancy-based zoning that changes with the furniture plan, or integration with a building management system. The choice is rarely absolute; many floors run both, with impulse relays handling fixed circuits and a bus segment handling the areas that need finer control.
This article is part of the Stoklink Impulse and Latching Relay Engineering Guide, and it sits alongside our other application notes on hotel and hospital corridor lighting and car park and stairwell circuits. Office lighting is a different problem from corridors: the zones are less predictable, the tenant changes layout more often, and someone eventually asks for dimming.
Why office lighting is the hard case for impulse relays
A corridor or stairwell has a fixed geometry. The switching points don't move once the building is commissioned. An office floor does move — partitions get relocated, desks get rearranged around a new team structure, and a meeting room gets carved out of what used to be open-plan. An impulse relay circuit wired to match the original layout doesn't adapt to that. It switches whatever loop of luminaires was landed on its output terminal, and if the furniture plan changes without a matching change to the lighting zones, people end up switching on lights over desks nobody occupies to reach the one bank they actually need lit.
This is the practical argument for bus-based systems in offices, and it's a legitimate one. It is not, however, an argument that impulse relays are the wrong device for offices in general — plenty of small offices, meeting rooms, and back-of-house areas never need re-zoning and are over-specified by a bus system that nobody in the building can reprogram without calling the integrator back.
Key takeaway: The deciding question for an office isn't "impulse relay or bus system" in the abstract — it's whether the lighting zones will stay fixed for the life of the fit-out. If they will, size impulse relays to the zones. If the client expects to re-zone with the furniture, budget for a bus system from the start.
How an impulse relay zone actually works on an office floor
Each impulse relay switches one group of luminaires from any number of momentary push-buttons wired in parallel to its control input. There's no controller, no bus cable, no addressing step — every button on that loop toggles the same zone. A 1–4 pole relay from a range like the Schneider Electric Acti9 iTL or ABB E290 fits a standard DIN board alongside the MCBs already protecting the circuit, and the coil draws current only for the duration of the pulse. Between pulses there is no holding current and no coil hum — a real consideration in an open-plan office where forty relays humming quietly behind a distribution board door is audible in a quiet afternoon.
The practical unit for planning is the zone, not the relay. Count the independently switched lighting groups on the floor plate — window-side row, core-side row, each meeting room, the corridor — and each becomes one relay output, sized in poles to the number of circuits or phases it needs to carry.
N_relays ≥ N_zones, with each relay's pole count matched to the circuits per zone
Where: N_zones is the count of independently switched lighting groups on the floor plate (window row, core row, each enclosed office, etc.), and pole count per relay is selected from the available 1–4 pole range so one relay covers one zone without splitting a zone across two devices.
What a bus-based system adds that a relay panel can't
A bus-based lighting system — KNX and DALI are the two an office fit-out is most likely to specify — puts an address on every luminaire or driver instead of grouping them physically at a relay output. That address is what makes re-zoning a software task instead of a rewiring task: move a desk cluster, reassign the luminaires above it to a different logical group, and the physical wiring never changes. It's also what makes dimming, daylight harvesting, and occupancy-linked scenes possible, because the controller can address one fixture at a time rather than switching a whole loop together.
None of that is available from an impulse relay. A relay is a two-state device — on or off — with no addressing and no data channel back to a controller beyond a basic auxiliary contact for state feedback. If the specification includes dimming to a target lux level, scene recall from a wall panel, or a BMS dashboard showing individual luminaire status, that's a bus job, not a relay job.
Bus-based lighting control
A wiring topology (KNX, DALI, or a proprietary equivalent) where luminaires, sensors and control panels share a common data bus and each device has an individually addressable identity. Grouping, dimming levels and scenes are configured in software rather than fixed by which physical loop a device sits on.
Zone (in relay-based lighting)
A fixed group of luminaires wired to a single impulse relay output and switched together as one unit. The zone boundary is physical — set by the wiring — not logical, so changing which luminaires belong to a zone means rewiring, not reconfiguring.
Wiring effort and cost, compared honestly
An impulse relay installation uses the same wiring practice as any other DIN-rail switching device: push-button loops in ordinary control cable, relay outputs feeding the lighting circuit, everything landed on standard terminals. There's no proprietary bus cable, no gateway, no commissioning software license, and an electrician who has never touched the specific relay range can still wire and test it from the terminal markings and a wiring diagram. That's a meaningful cost and schedule advantage on a straightforward fit-out.
A bus system adds a bus cable run to every device, an addressing and commissioning pass done with vendor software, and — usually — an integrator on call for future changes rather than the building's own maintenance electrician. On a large or reconfigurable floor that cost buys real flexibility. On a small fixed-layout office suite it mostly buys unused capability.
| Factor | Impulse relay zoning | Bus-based system |
|---|---|---|
| Re-zoning after a layout change | Rewiring required | Software reconfiguration |
| Dimming / scene control | Not available (on/off only) | Native, per-luminaire |
| Commissioning skill needed | Standard electrical wiring | Vendor software / integrator |
| Standby power draw | None (latching, no holding current) | Bus devices typically draw a small standing current |
| BMS / status feedback | Basic, via auxiliary contact only | Rich, per-device status and diagnostics |
| Cost on a fixed small floor | Lower | Higher, often disproportionate |
Key takeaway: A bus system's cost is mostly justified by re-zoning flexibility and dimming — not by the switching function itself. If the office doesn't need either, the extra cost buys capability nobody will use.
Where the two approaches meet: hybrid office panels
Most mid-size office fit-outs Stoklink customers specify don't pick one approach exclusively. Fixed-function circuits — corridor lighting, back-of-house stores, plant rooms, stairwells — get impulse relays because there's nothing to re-zone and no dimming requirement. Open-plan floors and meeting rooms, where the client has said the layout will change within a few years or wants daylight-linked dimming, get a bus segment. The two coexist on the same distribution board without conflict, because an impulse relay circuit and a bus device are electrically independent — the relay just needs its own protective device and control loop, same as it would in an all-relay installation.
One office refurbishment worth remembering as a cautionary case: a floor had been fitted years earlier with impulse relays zoned to match an open desk layout, three relays covering three roughly equal bands of luminaires. The tenant later split the space into six enclosed offices with partition walls that ran straight across two of those zones. Every partitioned room ended up sharing a light switch with its neighbour, because the wiring boundary and the new room boundary didn't line up. That's exactly the failure mode a bus-based system would have avoided.
Key takeaway: Ask about the client's re-partitioning plans before the zoning design is finalised. An impulse relay panel wired to today's layout is a liability if the layout is expected to change; it's a non-issue if it isn't.
Auxiliary contacts and status feedback
Impulse relays with an auxiliary contact or status feedback output can report their on/off state to a building management system without a full bus retrofit — a middle option worth knowing about before assuming the only choices are "plain relay" or "full bus". An interface or coupling relay can also sit between an impulse relay circuit and a BMS input card where signal isolation or voltage matching is needed; see our interface and coupling relay guide for how that pairing works, and the monitoring relay guide for supervising the circuits feeding the panel itself. It's a smaller integration step than a bus system, and it covers the common office requirement of "just show me which floor still has lights on at 7pm" without redesigning the switching topology.
For fit-outs that also need staircase-style timing on some circuits — a meeting room that should switch itself off after a set period, for instance — an impulse relay with an added timer function covers that without pulling in bus logic at all; see our timing relay guide for the wider timing-relay family.
Conclusion
Office lighting control isn't a single decision applied uniformly across a floor — it's a zone-by-zone judgement. Fixed circuits with no re-zoning or dimming requirement are a straightforward impulse relay job, cheaper to install and simpler to maintain than a bus segment would be for the same function. Open-plan areas with an expected layout change, or any requirement for dimming, scenes or per-luminaire BMS status, belong on a bus. Specify the two approaches by zone rather than picking one for the whole floor, and confirm the client's re-partitioning plans before the wiring goes in — that single question prevents the most common office lighting rework.
Frequently Asked Questions
Can impulse relays and a KNX or DALI bus system be used on the same office floor?
Yes, and it's common practice. Fixed-function circuits such as corridors, plant rooms and stores are wired to impulse relays, while open-plan or dimmable areas run on the bus segment. The two are electrically independent on the distribution board, each with its own protective device, so there's no conflict combining them on one floor.
Do impulse relays support dimming for office lighting?
No. An impulse relay is a two-state switching device — on or off — with no dimming output. If any part of the office requires dimming, daylight-linked light levels, or scene recall, that circuit needs a dimmable driver on a bus-based or 0-10V/DALI control system instead of a relay.
Why do offices sometimes end up with lights on over empty desks?
Usually because the impulse relay zoning was wired to match an earlier furniture layout that no longer matches the current one. Since a relay zone boundary is physical, not logical, a partition wall or desk cluster added later can straddle two zones, forcing occupants to switch on more of the floor than they need.
What does an impulse relay's auxiliary contact do for office BMS integration?
It reports the relay's current on/off state as a separate signal, which a building management system can read without needing full bus communication. It's a lighter-weight way to get basic status visibility into a BMS than deploying a full KNX or DALI segment purely for monitoring.
Is a bus-based system always worth the extra cost over impulse relays in an office?
Not always. The extra cost buys re-zoning flexibility, dimming and rich status feedback — real value on a floor expected to be reconfigured or on any space needing dimming. On a small office suite with a fixed layout and no dimming requirement, that cost is mostly paying for capability that never gets used.
How many impulse relays does a typical open-plan office zone need?
One relay per independently switched lighting group, sized in poles to the circuits it carries — a straightforward count of window-side rows, core-side rows, meeting rooms and similar zones on the floor plate, not a calculation tied to headcount or floor area.