SmartGlass dimming in a Hennur home office: why the wiring conduit placement breaks the light-control spec

A 3.2-metre west-facing window in a home office overlooking Hennur Main Road catches afternoon sun at 2 p.m., and the architect specified switchable glass to manage glare. The dimming response time was promised at 500 milliseconds. On handover, the glass took 2.5 seconds to transition from clear to tinted—five times slower than spec. The cause was not the glass itself, but 4.8 metres of surface-mounted electrical conduit that the contractor ran post-glass installation, adding capacitive load to the control circuit and introducing a lag that no amount of commissioning could fix.

This is not a failure of the glass. It is a failure of coordination. SmartGlass dimming—whether switchable bathroom privacy, conference room partitions, or overhead PDLC panels—depends entirely on electrical signal integrity from the controller to the glass itself. Conduit placement, wire gauge, earthing, and junction-box location are structural decisions that must be locked into the RCP and coordinated across MEP disciplines before the glass arrives on site.

The physics of tint lag: why conduit placement matters

SmartGlass uses a polymer dispersed liquid crystal (PDLC) layer that responds to electrical voltage. When 60 volts AC is applied across the layer, the crystals align and the glass clears. When voltage drops, the crystals scatter and the glass tints. The transition speed depends on the quality and stability of the electrical signal reaching the glass.

Surface-mounted conduit introduces two problems. First, unshielded runs of wire in open conduit pick up electromagnetic noise from adjacent power circuits—particularly problematic in Bangalore homes where MEP contractors often run lighting and AC circuits in parallel. This noise creates signal jitter that the controller must filter out, adding latency. Second, surface conduit has higher capacitance per metre than in-wall conduit because it sits further from earth reference planes. A 5-metre run of surface conduit can add 80–120 picofarads of stray capacitance. For a PDLC circuit operating at 60 Hz, this translates directly to response-time lag of 400–800 milliseconds.

In-wall conduit, by contrast, runs within the concrete structure and benefits from the building fabric as an earth plane. The same 5-metre run has 20–30 picofarads of capacitance. The difference is not theoretical: it shows up in real time as a visible delay between the moment a resident presses the dimming button and the moment the glass responds.

The Hennur case: what went wrong in the RCP

The architect's RCP showed SmartGlass at the west window, with a notation "electrical by contractor." The electrical consultant's drawing showed a 32-amp sub-board located 6 metres away, in the corridor outside the office. No conduit routing was specified. No controller location was marked. During construction, the contractor chose the path of least resistance: surface-mounted conduit from the sub-board, across the wall, and down to a junction box mounted 1.2 metres below the glass header.

When the glass was installed and commissioned, the tint lag became apparent immediately. The architect requested a retest. The glass supplier confirmed the panel was performing within spec (500 ms at the controller terminals). The electrical contractor was asked to re-route the conduit in-wall. The cost to cut chases, install in-wall conduit, and patch the facade was approximately 1.8 times the cost of the original surface run. The project overran by 12 days.

The lesson is simple: conduit routing is not incidental. It is a specification that must be locked into the RCP before any contractor mobilizes on site.

Coordination checklist: what to spec before the glass order

Controller location

Decide whether the SmartGlass controller will be wall-mounted or ceiling-mounted, and at what distance from the glass. A controller mounted within 2 metres of the glass panel, with in-wall conduit, will deliver the fastest response. Mark the exact location on the RCP with a 50 mm x 50 mm box symbol and label it "SmartGlass Controller—3A, 230V single-phase."

Conduit routing and earthing

Specify in-wall conduit (20 mm HDPE or equivalent) from the controller to the glass header. Mark the route on the RCP with a dashed line and note the conduit type. Require that the conduit be run through the building structure, not surface-mounted. If surface mounting is unavoidable (e.g., on a false ceiling), specify shielded conduit and require earthing at both ends. Earthing must connect to the building's main earth bar, not to a local earth stake.

Wire specification

The control signal from the controller to the glass must run in dedicated conduit, separate from power circuits. Specify 1.5 mm² twisted-pair shielded cable (Cat.5e or equivalent). Do not allow this cable to run in the same conduit as 32-amp or 63-amp power circuits. If separation is impossible, use double-shielded cable and increase the earth cross-section to 2.5 mm².

Junction boxes and termination

Mark all junction-box locations on the RCP. The controller should terminate within a wall-mounted enclosure (IP54 minimum) located at a height of 1.2–1.5 metres for accessibility. The glass-side termination should be a sealed junction box mounted at the header, not exposed on the glass frame.

Testing protocol

Include in the specifications: "SmartGlass response time shall be measured at the glass panel with a calibrated light meter and a stopwatch, from the moment the control button is pressed to the moment the glass reaches 90% of final tint. Response time shall not exceed 500 milliseconds. Testing shall be carried out with all building systems (HVAC, lighting, power distribution) operational at full load."

Retrofit scenarios: when conduit placement becomes critical

Many Bangalore homes, particularly in Whitefield and HSR Layout, are retrofitting SmartGlass into existing facades. In these cases, in-wall conduit is often impossible because the walls are already finished. The solution is to use surface-mounted conduit, but with three non-negotiable conditions: shielded conduit, earthing at both ends, and a controller mounted as close as possible to the glass (ideally within 1.5 metres).

If the controller must be mounted more than 3 metres from the glass, consider using a local relay module mounted at the glass header, with a low-voltage signal cable running from the main controller. This isolates the long run from the high-frequency noise on the control signal.

For retrofit smart films applied to existing glazing, the electrical challenge is even tighter because the film must be powered through the glass frame, and frame cavities are often constrained. In these cases, specify the smallest possible gauge of shielded wire (0.75 mm²) and run it in a 12 mm conduit with adequate earthing straps every 600 mm.

Commissioning and as-built documentation

Once the glass is installed and the conduit is in place, the commissioning process must include a formal test of response time. This should be documented in the as-built RCP with actual conduit routing, wire gauges, and controller location marked to the millimetre. If response time exceeds the spec by more than 10%, the cause must be investigated before the project is handed over. Common causes are incorrect wire gauge, earthing faults, or interference from adjacent electrical circuits.

Request from the electrical contractor a signed certificate confirming: conduit type and length, wire gauge and shielding, earthing resistance (should be less than 5 ohms), and the date of continuity testing. These documents become part of the building's MEP handover pack and are essential for future maintenance or modifications.

Questions we get asked

Can I run the SmartGlass control cable in the same conduit as power cables?

Not without consequences. Power cables generate electromagnetic fields that couple into unshielded control cables, introducing noise that delays the dimming response. If you must share conduit, use shielded cable and increase the earth cross-section. Even then, expect a 200–400 millisecond increase in lag. Dedicated conduit is always preferable.

What's the maximum distance from the controller to the glass?

In-wall conduit: up to 10 metres with no significant lag. Surface-mounted shielded conduit: up to 5 metres. Beyond 5 metres with surface conduit, response time degrades noticeably. If your site requires a longer run, use a relay module at the glass end.

Does the type of glass thickness affect response time?

No. PDLC response time is determined by the polymer layer and the electrical signal, not by the glass thickness. A 6 mm laminated PDLC panel and a 10 mm tempered PDLC panel will have identical response times if they receive the same electrical signal. The variable is always the conduit and wiring.

Our architect didn't mark the conduit route on the RCP. What do we do now?

Before the contractor mobilizes, issue an RCP addendum specifying: controller location, conduit type (in-wall or shielded surface), earthing requirements, and response-time test protocol. Have the electrical consultant sign off. This takes two days and costs far less than a retrofit conduit re-route on-site.

We're retrofitting SmartGlass into a flat in Koramangala with finished walls. What's the best approach?

Surface-mounted shielded conduit, controller mounted within 1.5 metres of the glass, and earthing at both ends. If the aesthetic is a concern, run the conduit behind a skirting board or within a decorative channel. The electrical performance will be acceptable, though response time may be 600–800 milliseconds rather than 500. This is still perceptible as a dimming action, not a lag.

Closing

SmartGlass dimming is a specification that touches electrical, structural, and architectural disciplines. A single RCP markup—the conduit route, the controller location, the earthing detail—prevents costly late-stage rewiring and ensures the glass performs as promised. Before you specify switchable glass, talk to the atelier and your electrical consultant together, with the RCP in front of you, to lock the conduit route into the plan. It takes an hour. It saves weeks.