Installation of a timer switch under the CS3 switch: step-by-step guide

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Installation of a timer switch under a switch CS3: step by step guide

The CS3 series from Elektrobock is among the most popular solutions for households where you want to automate lighting, a bathroom fan, a water heater, or other electrical appliances – without the need to interfere with the distribution board or replace the entire switch. The principle is simple: the switch is mounted under a standard switch frame, occupying space in the installation box. Despite being a relatively simple operation, I repeatedly see the same mistakes in practice – wire swapping, underdimensioned wires, poor cable arrangement in the box, or neglecting the depth of the installation box. This article will guide you through the entire process from the first opening of the box to the first programming, including answers to the most frequently asked questions by customers.

What is CS3 and why install it "under the switch"

The CS3 series includes several variants: CS3-1 (single-channel, 16 A, mechanical setting), CS3-1B (single-channel with display backlight), CS3-2 (dual-channel – controls two independent loads) and CS3-4 (four channels). All fit into a standard installation box with a diameter of 68 mm and a minimum depth of 42 mm. This feature – installation into an existing box – is a key advantage compared to DIN rail switches. If you want to compare both concepts, I recommend reading the article Timer switch under the switch vs. on DIN rail: which type to choose, where you will find a detailed analysis of advantages and disadvantages.

Installation "under the switch" means that the final appearance on the wall will almost not change – instead of a single switch, you will see a combination: a timer switch (or its display) at the bottom and a retained switch at the top for manual control. The frame remains the same, and the mounting frame of the CS3 switch fully fills it.

Tools and materials needed before starting

Before you reach for the screwdriver, prepare the following. You will need: a flat screwdriver (2.5 mm and 4 mm), a crosshead screwdriver PH2, an electrical voltage tester (contact or non-contact – both work), combination pliers, wire stripping pliers, and optionally an ohmmeter for checking connections. I recommend having a flashlight or headlamp handy – visibility in the box is often poor.

Materials may include cable CYKY 3×1.5 mm² or 3×2.5 mm² (depending on the connected load), WAGO 221 connectors or classic screw terminals, and possibly insulating tape. Important: if you need to run multiple wires to the devices with CS3-2 or CS3-4, check that you have enough space in the box. The dual-channel CS3-2 has more terminals and cables, which really take up space.

Step 1: Turn off the power and verify no voltage

This is a point where you should never compromise. Find the circuit breaker or fuse in the distribution board that powers the circuit and turn it off. Then return to the switch and use a non-contact tester to verify that there is no voltage on any wire in the box. Note: there may be multiple circuits in the box – for example, if there are multiple switches in a shared frame. I recommend always disconnecting the entire room, or even the bathroom, and checking each wire separately.

Very important practical warning: in renovated apartments, it is common for the phase wire to be routed differently than you would expect based on the insulation color. I have seen the blue wire (usually neutral) connected as a phase multiple times. Therefore, use a tester, not assumptions.

Step 2: Disassemble the original switch

After turning off the power, remove the frame (usually it can be pushed off by hand, or with a flat screwdriver). Underneath, you will find the mounting plate of the switch attached to the installation box with two screws – unscrew them. Carefully pull out the switch, without pushing it – the wires are often short and forceful pulling can break the connections. If the wires are too short (less than 8–10 cm from the edge of the box), note this – you will need to extend the wires using connectors.

Take a photo of the original switch wiring before disconnecting anything. It takes 30 seconds and can save you an hour of confusion when restoring the original state if you decide to replace the switch back.

Step 3: Identifying wires in the box

In a standard installation, you will find the following wires in the switch box: live (L) – brown or black, neutral (N) – blue, protective (PE) – yellow-green. In older installations, the coloring may not match the standard, so again – the tester is the law.

Important: in a lighting switch, the power cable sometimes runs through the luminaire and only the switched live and the neutral wire come into the switch box. This so-called "cross-wiring" method of connection is problematic for some variants of CS3, because the switch requires direct 230 V power supply (L + N) for its operation. If you have only two wires in the box (L and L-switched, without N), you will have to rewire the installation or use another solution. This occurs very often in older apartments from the 80s – roughly in every third apartment.

Step 4: Preparing the wires and checking the depth of the box

Before installing the CS3 itself, measure the depth of the box. A standard plastic box embedded in the wall has a depth of 40–42 mm. CS3 with wiring requires at least 42 mm, ideally 50–55 mm. If you have a shallow box (an older type with a depth of 35 mm), the CS3 will not physically fit without extending the box or using an adapter. This is the most common physical problem during installation – customers often underestimate it.

Strip the insulation from the wires to the length specified by the manufacturer – usually 6–8 mm. A wire stripped too short will not stay in the terminal, and a wire stripped too long is a safety risk (exposed live wire). Before inserting into the terminal, make sure there are no loose strands (in stranded wires), or use ferrules. CS3 uses screw terminals, so tighten each terminal to the value specified in the manual – usually 0.5–0.6 Nm. Over-tightening can damage the wire, while under-tightening causes transition resistance and heating.

Step 5: Connecting the CS3 terminals

We are now reaching the core of the entire installation. CS3 has clearly labeled terminals on its body – for each model, the wiring diagram is printed directly on the cover or in the manual. Always refer to the provided diagram, not from memory or third-party internet instructions, because variants of CS3-1, CS3-1B, CS3-2 and CS3-4 have different terminal blocks.

Basic terminals common to all models:

L (IN) – here we connect the live wire from the network (brown/black). This is the power input.
N – neutral wire (blue). The switch needs N for its own electronics power supply.
L (OUT) or channel number (1, 2...) – the output switched live wire leading to the load.
Some versions also have a terminal for a switching input (manual button) – if you are keeping the original switch, connect its contacts here.

Connecting the terminals step-by-step:

First, connect the neutral wire (N) – this is a safer sequence, since N is not under voltage when the circuit breaker is off.
Connect PE (protective conductor), if required by the load – it runs directly through or alongside the switch.
Connect the power live wire L (IN).
Connect the output live wire L (OUT) leading to the load.
If you are keeping the manual switch (recommended), connect its contacts according to the diagram.

In the model CS3-2, you have two output terminals for two independent circuits – for example, lighting in the entrance and lighting in the hallway. Each channel is programmed separately. Be careful with the total maximum current: the sum of the loads on both channels must not exceed the maximum rating of the switch (usually 16 A for resistive loads). More information about power limits can be found in the article What is the maximum power a timer switch or Elektrobock socket can handle?.

Step 6: Storing the wiring and physically mounting the CS3 into the box

After connecting all the terminals, it is time for one of the most important craft steps – properly placing the wires into the box. The wires must not be in loose loops, nor too tight. Ideally, fold them into a flat loop (or "figure-eight") in the lower part of the box, with the CS3 module placed over them. The wires must remain under the module, not on the sides, where they could be pinched by screws or the edge of the box.

CS3 is mounted in the box with two screws M3,5 or M4 into plastic stands – just like the original switch. Tighten the screws evenly, so that the module is in one plane with the wall. Over-tightening can damage the box (plastic stands can be pulled out), while under-tightened screws will allow the switch to "float" in the frame.

Finally, install the decorative frame of the CS3 – it will snap into the mounting part of the switch. The CS3 frame is standardized to the size of Legrand Valena / ABB / other compatible frames. Check before purchase whether your existing frame fits – most customers have no problem, but for some regional brands, you may need to replace the frame.

Step 7: First Power-Up and Functionality Check

Before switching on the circuit breaker, visually check once again:

No bare conductor is sticking out of the terminal.
The frame is evenly mounted on the wall.
Module CS3 is not wobbling in the box.
No conductor is caught between the module and the edge of the box.

Switch on the circuit breaker. CS3 will initialize – on the display (for variants with a display), the current time will appear or a blinking cursor will indicate the need for setting. For mechanical variants (CS3-1 without a display), check whether the switching wheel rotates freely and whether voltage is present on the load in the "on" position (check with a tester or simply – the light will turn on).

If the switch does not start, the load does not respond or the display remains dark, switch off the circuit breaker and check the wiring. The most common error: swapped L and N, or N not connected (the module has no power for its electronics). Another common error: the output terminal L-OUT remains unconnected, because the customer forgot to run a wire from it directly to the load (relied on the original wiring, which however did not go through the output terminal of CS3).

Step 8: Setting the Time and Programming Switching Events

After a successful start-up, proceed to set the current time and date (for weekly programs also the day of the week). The procedure depends on the specific model – CS3-1 has a simple mechanical setting using a wheel, while CS3-1B and other digital variants have buttons for HOUR, MIN, DAY and MODE. A detailed guide to programming is available in the article Programming a Weekly Switching Program on Elektrobock Switches.

In practice, I most often program these scenarios:

Morning Wake-up: turning on the bathroom light or fan at 6:30, turning it off at 7:30.
Security Lighting: turning on the outdoor light at 18:00, turning it off at 23:00 every day.
Water Heater: turning on the water heater at night at 23:00 (cheaper night tariff), turning it off at 5:00 – the customer has enough hot water in the morning and pays the night rate.
Aquarium: alternating light over the aquarium according to the biological cycle.

If you need to control the load wirelessly or remotely, consider combining CS3 with a remotely controlled socket WS101 for other circuits where a socket solution is sufficient. WS101 does not require any changes to the installation and is suitable for portable appliances.

Common Installation Errors with CS3 and How to Avoid Them

Over the years of practice, I have seen several recurring errors during the installation of CS3. Here are the most important ones:

Shallow installation box: as mentioned, the minimum depth is 42 mm. Solution: replace the box with a deeper one (type KPR 68-3 or similar) or use an extension mount.
Missing neutral conductor in the box: CS3 needs N for its own power supply. If N is not present, a new cable must be run – always cable CYKY 3×1,5, not just one conductor.
Overloading the output: customers sometimes connect too large a load to one channel of CS3 – for example, a 3 kW electric heater directly. CS3-1 can handle 16 A resistive load (i.e., ~3 680 W), but only with the correct cable cross-section of 2.5 mm². Never exceed the rated values of the switch. Details on dimensioning can be found in the article What maximum power can a time switch or Elektrobock socket handle.
Swapped L and N on the input terminals: the switch usually "somehow" works even with the swap (the electronics are protected), but it is a safety risk and can lead to relay failure.
Missing manual bypass: in a correct installation, a manual switch should be preserved as an override. If you connect it according to the scheme on the corresponding CS3 terminal (S/INT input), you can turn the light on or off at any time without changing the program.
Loose terminals: a loose terminal is a source of heat and fire. Check each terminal by gently pulling the wire – it must not slip out.

Differences between variants CS3-1, CS3-1B, CS3-2 and CS3-4

To choose the right model, here is a quick comparison:

Model	Channels	Display	Max. current	Typical use
CS3-1	1	Without / mech.	16 A	Light, water heater, fan
CS3-1B	1	LCD + backlit	16 A	As CS3-1, better readability
CS3-2	2	LCD	16 A / channel	Two lights, light + fan
CS3-4	4	LCD	16 A / channel	Complex automation, 4 loads

If you are undecided between models, read the article How to choose the right Elektrobock timer for your home, where you will find a more detailed analysis according to the type of load and programming requirements.

Practical examples from real customer orders

Order 1 – apartment building, bathroom fan: The customer had a fan in the bathroom connected to the light switch – it started only when the light was on. He wanted it to run for 15 minutes after the light was turned off. Solution: CS3-1 in a box with retained control of the light via a bypass. The fan was connected to the output channel of the CS3. Problem we solved: the neutral conductor was missing in the bathroom box. We had to run a new CYKY 3×1.5 from the distribution board – an additional half-day of work, which the customer did not expect.

Order 2 – family house, outdoor lighting: The customer had three outdoor lights controlled by one switch at the entrance. He wanted automatic switching on at dusk and switching off at 23:00. We chose CS3-1 (single-channel), with all three lights connected in parallel to one output. Total power 3× 11 W (LED) = 33 W – well below the limit. Installation took 45 minutes including programming. The customer was satisfied, but after a month he called saying that "the clock is going backwards". It turned out that we had forgotten to synchronize daylight saving time during the first installation. Since then, I always remind customers to check after the switch to summer/winter time.

Order 3 – cabin, water heating and lighting at the same time: The cabin owner wanted to control the water heater and the outdoor light from one place according to different time programs. An ideal opportunity for CS3-2. Both channels were programmed independently: water heater from 20:00 to 6:00, light from 19:00 to 23:30. The box had a depth of only 40 mm – edge to edge. We had to lay the wires very carefully, but it fit. I recommend replacing it with a deeper box for every new installation, even if it "just fits".

Safety rules that must always be followed

To conclude the technical part, a few rules that apply without exception:

Never work under voltage – not even "just for a moment to check". Not even once.
Use a protective device (RCBO, RCD) on the circuit – CS3 is not a protective device, only a switching element.
Do not forget the PE conductor for loads where it is required (Class I appliances).
Installation in the bathroom must respect the protection zones according to STN EN 60364-7-701 – install CS3 only in permitted zones.
After each installation, perform a visual inspection and functional test before handing over the job.
Keep the manual and wiring diagram with the device itself (stick it in the distribution board or attach it to the building documentation).

Most frequently asked questions (FAQ)

Do I have to replace the entire installation box, or is the old one sufficient?

It depends on the depth of the original box. If you have a standard 42 mm box and the cabling is not too long, the CS3 will fit. If you have an older shallow box (35 mm or less), replacing it with a deeper one is necessary. Replacing the box takes an experienced electrician 20–30 minutes and is a worthwhile investment – you will avoid problems with overheating of compressed wires.

Can I install the CS3 myself, or must an electrician install it?

According to current Slovak legislation (Act No. 124/2006 Coll. and Decree 508/2009 Coll.), only a person with professional qualifications (an electrician with the appropriate qualification) may perform electrical installations in fixed wiring. Technically, the installation of CS3 is simple, but if you are not an electrician, I recommend leaving it to a professional. Improper installation can be a reason for the rejection of insurance coverage in case of fire.

CS3 reset after a power outage – I lost my program. What should I do?

CS3 has an internal backup capacitor or battery (depending on the specific model), which retains time and program during short outages. However, during a longer outage (several days, e.g., when the cabin is turned off), the time may be lost. The event program is usually stored in memory – only the current time is lost. After power is restored, it is sufficient to set the correct time. If the entire program is reset, it is a fault of the backup power source – the solution is to replace the battery (if replaceable) or to service it. More about typical faults can be found in the article Common faults of Elektrobock timers and their solutions.

Can I connect LED lighting to the output of CS3-1? Won't it "flicker"?

Yes, CS3-1 is compatible with LED loads. The relay in CS3 is mechanical, so unlike some dimmers, there is no problem with flickering. The only exception would be if the LED driver was very sensitive to current spikes at switching on – but this is extremely rare. In practice, I have not encountered any case in years where standard LED lights with CS3 caused problems. Minimum load is not a limiting factor for CS3 (the relay works even with a single 3 W LED bulb).

What is the difference between CS3 and CS4 – when to choose which?

CS3 is intended for installation in an installation box under a switch – that is, directly on the wall, without the need for a distribution board. CS4 is a DIN rail timer, which is mounted in an electrical distribution board (home distribution board). CS4 can handle higher currents and is more suitable for central control of larger loads. If you want to automate a single light or water heater without interfering with the distribution board, CS3 is the right choice. If you are dealing with multiple circuits centrally, or have a load exceeding 16 A, go for CS4. A detailed comparison can be found in the article Wall-mounted timer vs. DIN rail timer: which type to choose, or in a separate article about installing CS4 on a DIN rail.

Can CS3 control an electric floor heating mat?

Yes, but with conditions. A floor heating mat is a resistive load – ideal from the perspective of CS3. A problem may arise with larger areas, where the combined power of the mat exceeds 16 A (3 680 W at 230 V). In that case, the CS3 must control only a contactor, not the mat directly. For areas up to about 25–30 m² with mats of 100–150 W/m², you usually stay under 16 A, but always calculate the total power and compare it with the rated values of the switch. For heated areas, I recommend cable CYKY 3×2.5 mm² instead of the minimum 1.5 mm².

Conclusion

Installation of a CS3 timer under a switch is a task that an experienced electrician can complete in 30–60 minutes. For a less experienced customer or DIY enthusiast, it is crucial to understand three things: first, never work under voltage; second, check the depth of the box in advance and make sure the neutral conductor is present; and third, always follow the wiring diagram of the specific model, not a general description from the internet. The CS3 series – whether CS3-1, CS3-1B, CS3-2 or CS3-4 – is a reliable, long-term tested device that serves without problems for years when installed correctly. If you are unsure about model selection or the procedure, check the other articles in our Knowledge Center – there you will find answers to most of the questions customers repeatedly encounter.

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