Secure AC Power

This article involves dangerous AC currents. Working with AC currents requires utmost caution and expertise.

Disclaimer and Safety Warnings

Warning

Handling AC currents can be extremely dangerous. Improper handling can result in serious injury or death. Ensure all safety precautions are taken and consult with a professional.

Using Insulated Housing

Always consider that not just you but anyone may be exposed to your device: kids, cleaning personnel, colleagues, family members.

Temporary setups are not ok for anything exposing live AC currents.

Always keep your prototypes disconnected from AC when not under your direct supervision until you enclose the electrical components within an insulated housing, ensuring all connections are secure and no live parts are exposed. Double-check the installation to make sure the housing is properly sealed and provides full coverage. Ensure there are no gaps that could expose wires or connections.

Disclaimer

The information provided in this article is for educational and informational purposes only. The author and publisher are not responsible for any injuries, damages, or losses resulting from the use or misuse of the information provided. Working with electricity is inherently dangerous and should only be performed by qualified professionals. Always follow proper safety procedures and consult a licensed electrician before attempting any electrical work.

Safety Guidelines

• Always turn off the power at the main circuit breaker before starting any electrical work.
• Use insulated tools and wear rubber-soled shoes to reduce the risk of electrical shock.
• Never work on electrical systems in wet or damp conditions.
• Verify that the power is off using a reliable voltage tester or multimeter.
• Do not touch live wires or components with bare hands.
• If unsure about any procedure, consult a licensed electrician.

Recently, a smoldering USB cable in the kitchen caught my attention. It was part of a USB mist humidifier, had a short circuit and was close to catching fire. Considering the density of devices of various origin and quality, power supplies and batteries, loose cables, and prototypes that can be found in our homes, let alone a hobbyist lab, it’s a great idea to (a) cut off AC power when the lab is unattended, and (b) install a smoke alarm (if you haven’t done so already).

A power meter and mains switch can secure AC power supply in environments like labs (also in many other scenarios). Securing AC power while a certain environment is not used or unattended can help save energy, but even more so prevent fire hazards.

Here are the requirement specifications:

• High Currents: the device controls the total power consumption of an entire lab (or when applied to other use cases: the charging loads for an electric car, or an entire area like the kitchen). In any respect, the device needs to safely handle dangerous voltages at high currents.
• Logging: Power consumption needs to be logged automatically so it can be analyzed in retrospect when power was used, and how much total power was consumed.
• Remote Access: Power supply must be cut off at any time from anywhere, including secure remote access (to check from abroad) since the device is supposed to cut off power for safety reasons whenever power is not required or the devices are unattended.

Obviously these specs are just an example and should be arbitrarily adjusted: cutting off power for the kitchen while away from home would probably turn out to be not the best of ideas once you peek into the refrigerator after returning home.

Power Metering And Switching

To ensure that power metering and power switching is safe, using a DIY solution is not wise: the parts cost more than ready-to-use and proven-to-be-safe devices.

Logging And Remote Access

Since part of the requirements is remote access, when choosing a WiFi- or Zigbee-enabled power monitoring component, you get these features for free, and the component can be remotely controlled via a vendor app. Often, this also includes logging.

A lot more features become available when you pick a vendor that is supported by any Home Assistant integration in which case the device can be easily added to Home Assistant.

This adds additional features for free:

• more advanced logging
• custom dashboards
• automation scripts to connect the power monitor features with other devices.

One of the most economic approaches is to look for devices that work with Tuya. You can pick from a wide variety of devices and get good hardware at extremely competitive prices.

Smart Plugs

The cheapest solution is to use a Tuya smart plug (under €3 per piece): it comes with a WiFi-enabled microcontroller, power metering, is rated for 220V 20A, and it can be integrated into Home Assistant via the official Tuya integration:

On inspection, smart plugs in this low price region use very simple mechanical relais that - despite their specs - do not seem fit enough to deal with the anticipated currents.

Rail Meters

Rail power meters are designed to be mounted to standard DIN 35mm rails inside fuse boxes. They are commonly used to measure and switch entire electrical segments.

The rail meter in this example is a Tuya-compliant Atorch GR2PWS rated at 50-320V AC/100A/32KW. There are similar devices from other vendors. They cost around €15-20.

Pay very close attention to the exact device model you order: most vendors also sell look-alike models with less features. You need a model that supports both WiFi and Switching. Also make sure the model has a Home Assistant integration, i.e. supported by Tuya.

Here are the different models for the Atorch device used in this example (along with feature-limited models you want to avoid):

Feature	GR2P	GR2PW	GR2PWS
Power Monitoring	yes	yes	yes
WiFi	missing	yes	yes
Switch	missing	missing	yes

Adding Power Meter

There are a few ways how you can add a power meter:

• Directly: connect the power meter directly to an AC line ensures that you have full control over L and N lines since a user cannot unplug and accidentally reverse these lines. The installation effort is higher, and the device isn’t portable anymore.
• Own Plugs: dedicated two-pin earthed plugs (“Schuko” in Germany for Schutzkontakt) make a device fully portable: plug it into an outlet on one side, and plug in a load or an extension cord on its other side. This introduces risks because each additional plug adds additional resistance which translates to heat, and with high currents results in fire hazards. It also requires you to add new cables, and when you pick cables with insufficient cross-sectional area, again this adds a fire hazard. That’s why using dedicated plugs is not recommended.
• Grind In: Take a good quality extension cord with the appropriate ratings, and cut its cord. Connect both ends of the cord to the power meter, and directly contact the Ground wires of both ends. This approach has the least risks attached as you are using only materials that are certified for the use case and just need to make sure you are connecting both ends of the wire properly.

Whether you should use extension cords or stationary electrical installations depends on the use case and the power requirements of the devices you use.

Wiring

Wiring is as simple as connecting a regular switch: the device comes with two screw terminals: N-IN and L-IN are connected to AC power, and N-OUT and L-OUT connect to the devices.

What does “N” and “L” mean?

AC power uses three lines: L (Live), N (Neutral), and Ground. According to IEC60446, the L-line should be marked by a brown cable, and the N-line is marked by a blue cable.

• The L-line is live (dangerous): when you connect an AC tester pen to it, its indicator lamp glows. Likewise, when you touch this line, you get an electrical shock.

  More likely, when you touch (or connect a device to) the live line, the RCD (residual current breaker) triggers and cuts off power. RCDs are mandatory in electrical installations in Europe but may be missing elsewhere or in very old installations.

• The N-line is neutral and works basically like the internal ground for the load and closes the circuit. Touching it causes no harm.

• The Ground line represents the innocent bystander-world, and modern electric installations monitor Ground and immediately disconnect power when it manages to make contact with Ground.

Always Connect Ground

That’s why it is very important to always carefully connect Ground. The power meter is only concerned about L and N and has no terminals for Ground (because Ground is not important for its job). Make sure you connect the Ground from the power supply plug to the Ground of your load.

Using Plugs Can Reverse “N” And “L”

• With fixed installations, you have control over L and N: make sure you connect your power supply live line to the L-IN terminal of the power meter.

• If you use plugs (i.e. extension cords), you have no control over how the user later plugs in the plug, and L and N may be reversed.

Most cheap switches contain only one switch, so only one side of the circuit can be interrupted. When you plug in your device in the wrong way and reverse L and N, then the switch may cut the N line instead of the L line.

The loads still stop working when you break N instead of L (since the circuit is interrupted), but now the load is always connected to the live line, and if there was a defect, or if a user would touch an uninsulated part of this live line, the user would get an electrical shock even though the power meter had turned off the circuit.

For practical purposes, this is not necessarily a problem: any extension cord with a switch has the same problem: when plugged in “the wrong way”, then the switch does not separate the loads from the live line (except if it uses a switch that always cuts both the N and the L wire.

The device created here is not likely to be plugged in and out all the time. So when you plug it in, make sure you plug it in the right way, connecting its live line with the live line of the socket. In order to be able to do this, make sure you mark the live line at the plug of your device.

Connecting Cables

When you connect cables, you may want to add cable shoes.

• L-IN (brown) and N-IN (blue) are connected to the plug.
• L-OUT (brown) and N-OUT (blue) are connected to the extension cord body.

• Ground (yellow) needs to be connected by yourself (always do connect ground for safety reasons):

  

• Add a pull relief of some kind to the cables, i.e. use cable binders that block the cables from being pulled out the housing.

  

Pairing And Programming

Once you connect the plug to AC power, the Atorch GR2PWS turns on its screen and starts its intialization. You can customize it via the three push buttons when asked to, but if you simply do nothing and enjoy a coffee for a few minutes, the power monitor boots fine with its default settings.

Local Control

If you stop here, the power monitor works in local mode: you can view the readings on its screen, and operate a simple menu via three push buttons.

The component even logs data already, so you can switch to a graph view and analyze past power consumption.

Adding Device To WiFi

To remotely control the power monitor, you need to connect it to your WiFi. During boot up, the component displays a QR code that you can scan with your smartphone to use its temporary access point, and provide it with your WiFi SSID and password.

Remote Control

If you’d like to later add the component to Home Assistant, or use the vendor app to control the component remotely from anywhere in the world, do not manually configure WiFi via the QR code. Instead, add the device to the Tuya or SmartLife app.

During this pairing, your WiFi access is shared with the device, and it appears in the app.

You can now view the current state, and use the button at the bottom of the screen to toggle the component between on and off. Since this uses the Tuya Cloud API, it already works at home and abroad.

By default, the component turns the load off. You need to at least press the power button in the app once to turn the load on. You hear a mechanical relais click when you do this.

Adding Device To Home Assistant

Once the device has been added to your Tuya or SmartLife app, it will be automatically imported into Home Assistant if you have enabled its Tuya Integration.

To refresh Tuya devices immediately, in Home Assistant go to Settings, then Devices & services, then click the Tuya integration. Next, click the three dot menu, and choose Reload. If you have any issues, you may as well reboot Home Assistant altogether.

Enabling Entities

By default, when the power meter is added to Home Assistant, you do not see all of its entities. For example, the entities for the current voltage and current are missing.

For some reason, Home Assistant generally disables many entities with smart switches and lets you only control the switch. You have to manually turn disabled entities on if you want to view the readings.

To enable entities, click them, then in the top of the page, click the gear icon.

Now, switch the entitiy to Enabled, then click UPDATE. It takes 30s until the newly enabled entity becomes available in Home Assistant. Do this for all disabled entities.

Once done, you can now use (almost) all entities and can for example add them to Home Assistant dashboards:

Viewing Total Power Consumption

There is one entity that cannot be enabled and in fact is missing in Home Assistant: the total power consumption.

Total power consumption is missing for all Tuya devices. It can by default only be viewed in the official vendor app.

To add total power consumption, and in fact to add all missing Tuya entities to your Tuya devices in Home Assistant, install the ingenious Xtended Tuya integration.

Housing

The rail meter is designed to be used inside a fuse box. When you use it elsewhere, a proper housing is required.

Design

Since the device is not in plain view for the use case discussed here, no sophisticated housing is required, and a simple protective housing can quickly be put together in Fusion360 or any other CAD software:

3D Printed Housing

To 3D print the housing, use the STL files below.

• Bottom: takes the Atorch GR2PWS component and provides room for friction cable pull relief (by using cable binders on both cable ends), and spared room for connecting both Ground wires.
• Cover: provides openings for ventilation, and an opening for the Atorch GR2PWS screen and buttons.

Use four M4 screws to secure both parts. You may want to use a drill to clean the first few millimeters of each hole from printing residue.

Before you close the housing, mark the L line at the plug. This way, for added safety you can plug it into a socket in such a way that the switch cuts the live line and not the neutral line.

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