SmartGlass dimming wiring through a Hennur partition: why surface-mounted conduit beats in-wall when retrofit relay placement breaks the tint uniformity spec

A 4mm laminate partition in a Hennur residence last month showed us something we've now seen three times in eighteen months: a retrofit dimming system wired through the wall cavity, with the relay buried 2.8 metres from the glass edge, delivered uneven tint response across the panel face. The architect had specified in-wall conduit to keep the junction clean. The electrical contractor routed it through the stud cavity. By handover, the left third of the glass stayed fractionally lighter than the right two-thirds when the dimming signal fired. Not a defect in the glass. A lag in the signal path.

This is not a story about aesthetics. It is a specification problem that sits at the intersection of electrical coordination and glass performance — and it matters because most retrofit dimming jobs in Bangalore go in-wall first, ask questions later.

Why signal lag shows up as tint uniformity failure

SmartGlass — whether electrochromic or PDLC film — responds to voltage. The response is not instantaneous. A 5-volt dimming signal travels from the relay to the conductive bus on the glass edge. On a standard 4mm panel, the bus runs the full perimeter. The voltage arrives at the corner nearest the relay first, then propagates around the edge at the speed of electrical current through the bus material — roughly 0.6c, or 180,000 kilometres per second in copper.

On a 1.2-metre-wide partition, that propagation takes microseconds. Not measurable by eye. But when the relay sits 2.8 metres away, routed through in-wall conduit with three 90-degree bends, the signal path adds capacitance. The wire gauge matters. The number of junctions matters. The presence of other live circuits in the same cavity matters. The result: a 15 to 40 millisecond delay between the nearest edge and the far edge of the glass.

To the human eye watching the glass dim, that delay reads as a colour gradient. The near side darkens first. The far side lags. If the relay is positioned badly — say, on the left wall of a corner partition — the left panel dims 30 milliseconds before the right. That is enough to see a visible tint band across the joint line. On a 2.4-metre-wide conference partition that disappears when dimmed, a tint band reads as a flaw.

The in-wall conduit problem in Bangalore retrofit jobs

Why architects specify it

In-wall routing is the default on retrofit jobs in Bangalore because it is the path of least resistance. The glass is already installed. The partition frame is closed. Running conduit on the surface means rerouting it, boxing it, or accepting visible cable trays. On a high-end residential job in Sadashivanagar or Indiranagar, visible conduit reads as unfinished. So the electrical contractor chases the wall, runs the conduit through the stud cavity, and terminates it at a concealed junction box behind the partition.

The architect's spec says "conduit to be concealed where possible." The contractor interprets that as "in-wall is the goal." Nobody asks whether the signal path is compromised.

What happens in the cavity

The Bangalore monsoon (June to September) brings humidity to 70–85 percent. The granite belt ensures hard water — Cauvery TDS runs 200–300 ppm — which means mineral deposits on any exposed metal. Inside a stud cavity, condensation pools. Even with damp-proof membrane, moisture creeps. If the conduit is not sealed at both ends, the relay box becomes a moisture trap.

More immediately: in-wall conduit shares the cavity with structural wiring, sometimes HVAC ducting, sometimes plumbing rough-in. Every junction, every crossing, every bend adds impedance to the signal. The relay sits at the end of a 3-metre run with four elbows. The glass sits 2.8 metres from the relay, but the electrical path is longer. Signal integrity drops. Tint uniformity breaks.

Surface-mounted conduit: the spec that protects glass performance

The alternative is to run conduit on the surface, from the relay to the glass edge, in a single straight run with zero bends. This requires coordination with the architect at the design stage — not retrofit thinking, but new-build thinking.

A surface-mounted run in brushed stainless steel or powder-coated mild steel does not disappear. But it reads as intentional. On a partition in a home office or study, it can be positioned on the rear face (the face the camera does not see in the open-plan). On a bathroom where dimming glass is specified — say, a switchable privacy panel — the conduit runs vertically from the switch to the glass, a clean line that reads as part of the electrical infrastructure, not an afterthought.

The signal path is direct. The relay can be mounted at the base of the partition, 0.3 to 0.5 metres from the glass edge. The wire gauge is 2.5mm² or larger. The run is unobstructed. Signal propagation is uniform across the glass face. Tint response is simultaneous. A 1.2-metre panel dims evenly in 120 milliseconds, not in a visible gradient.

Relay placement as a specification variable

Most architects do not think of relay placement as a specification variable. The electrical contractor places it where the power source is. On a retrofit job, that is often the nearest wall or the closest existing switchboard. On a new-build job, the relay can be specified to sit within 0.5 metres of the glass edge, mounted on a discrete bracket, fed from a dedicated circuit.

If the relay must sit more than 1.5 metres from the glass (say, because the switchboard is on the opposite wall), then surface-mounted conduit becomes mandatory. In-wall routing will degrade performance. The specification should read: "Relay to be mounted within 0.5 metres of the glass edge, fed via surface-mounted conduit, 2.5mm² cable, direct run, no bends. If relay position exceeds 1.5 metres from glass, specify retrofit smart film with integrated driver to eliminate the signal path variable."

On the three Bangalore jobs where we've seen this issue, the relay was placed 2.8, 3.2, and 4.1 metres from the glass. None of the architects had specified relay position. All three had specified "conduit to be concealed." The electrical contractors had done what was easiest, not what was right for the glass.

Coordination language for your next spec

If you are specifying dimming glass — whether clear-to-blackout electrochromic or PDLC film — add these lines to your electrical coordination notes:

• Relay position: specify a maximum distance from the glass edge (0.5 m recommended).
• Conduit routing: specify surface-mounted on new-build jobs; if in-wall is required on retrofit, mandate a straight run with no more than one 90-degree bend, and seal both ends against moisture.
• Cable gauge: specify 2.5mm² minimum for runs over 5 metres; 1.5mm² for runs under 3 metres.
• Signal testing: require the electrical contractor to measure voltage at the glass terminal before and after the relay fires, to confirm no more than 0.3V drop.
• Handover: require a shop drawing showing relay location, conduit routing, and cable gauge, signed off by both the electrical contractor and the glass atelier before installation.

This is not standard practice in Bangalore yet. Most electrical contractors will push back. They will say "conduit is conduit" and "the relay works fine." They are right that the relay works. They are wrong that the glass performance is unaffected. The difference between a tint-uniform panel and a tint-banded panel is not a defect in the glass. It is a failure of electrical coordination to account for signal integrity.

Questions we get asked

Can we fix tint banding after the glass is installed?

Not easily. If the relay is buried in the wall, you would need to expose the conduit, shorten the signal path, or replace the relay with one closer to the glass. On a finished interior, that means cutting into plaster, relocating the junction box, and re-testing. It is cheaper to specify correctly the first time.

Does the cable gauge really matter that much?

Yes. A 1.0mm² cable over a 3-metre run with four bends will show measurable voltage drop — typically 0.4 to 0.6V on a 5V signal. That is 8–12 percent loss. The glass will dim, but not uniformly. A 2.5mm² cable over the same run will drop 0.1V or less. Specify by distance, not by "standard practice."

What if the switchboard is on the opposite side of the house?

Then you have three options: (1) run surface-mounted conduit the full distance and accept the visual; (2) relocate the relay to a position closer to the glass, fed from a sub-circuit; or (3) specify smart film with an integrated driver, which eliminates the external relay and signal path altogether. Option 3 is cleanest, but the film cost is higher. Option 2 is the spec compromise: a small relay box mounted on the partition frame itself, fed from the main switchboard via a single conduit run.

Does monsoon humidity really affect the signal?

Not the signal itself, but it does affect the relay and junction box. Moisture in the stud cavity corrodes contacts and increases impedance. We've seen relay failures in Bangalore homes during the monsoon season (June to September) on in-wall installations that were fine in the dry season. Sealed, surface-mounted conduit eliminates that risk.

Is this a problem with all SmartGlass, or just electrochromic?

Both electrochromic and PDLC film are voltage-responsive. Both show tint banding if the signal path is uneven. The effect is more visible on electrochromic because the tint change is gradual — you see the gradient form. On PDLC, the change is faster, so the banding is less obvious, but it is still there if you look closely at the panel face during dimming.

Commissioning your next dimming glass spec

If you have a partition or window that needs dimming control, and you want the tint response to be uniform across the glass face, talk to the atelier before you finalize the electrical coordination. We can review your relay position, conduit routing, and cable spec, and flag any risks to glass performance. The conversation takes 20 minutes. The cost of getting it wrong is a visible tint band and a retrofit fix that eats into your schedule and your margin.