Monitoring Power Distribution Systems

Using Low Voltage Circuit Breaker Trip Units


By Utility Relay Company

More and more facilities are discovering the benefits of monitoring power distribution systems. In this article, we will address power monitoring system basics, benefits and requirements. Additionally, we’ll outline a simple example system, which can even be applied to existing equipment.

What is it?Power monitoring system diagram | Monitoring Power Distribution Systems | National Field Services

A typical power monitoring system often allows the operator to monitor at least some of the following basic items:
• Metered values (e.g. current, voltage, power, energy)
• System switching configuration / status (e.g. breaker positions)
• Events and alarms


What are these systems used for?

Companies finding the most benefit from these systems are using them for the following:
• Verification of electrical service reliability, quality and continuity
• Monitoring expected process electrical loading
• Increasing safety by monitoring and operating devices without exposing personnel to shock and arc flash hazards
• Keeping track of maintenance information and records
• Realizing financial benefits through energy measurement

How does it work?Human Machine Interface (HMI) diagram | Monitoring Power Distribution Systems | National Field Services

Typically, a parent device requests and obtains communications data from multiple child devices. The data is interpreted and can be displayed on Human Machine Interfaces (HMIs). Low Voltage Circuit Breaker Trip units typically utilize RS-485 (serial) communications networks (twisted-shielded pair, daisy-chained from device to device).

Trip units are the “child” devices, and the trip unit network is wired to a “parent” device, which interprets the data. The parent device must have a driver for each device type that it is connected to. Each trip unit must be programmed with a unique address, to allow the parent device to identify each trip unit correctly. The parent device must be capable of the same communications protocol (language) as the trip units.

What is a communication protocol?

A communication protocol is a system of rules that allow two or more devices to transmit and receive information with each other. AC-PRO® and AC-PRO-II® trip units utilize the industry standard Modbus RTU communications protocol. Multiple communications protocols cannot successfully coexist on the same local RS-485 network.

Some examples of legacy OEM trip unit proprietary protocols are Commnet and INCOM. These trip units typically cannot be placed on the same local RS-485 network as Modbus RTU devices. However, in some applications converters are available.

Typical Power Monitoring System Requirements

Parent Equipment collects and/or displays the system information. Typical HMI/parent equipment include computers, servers, displays and graphical user interface (GUI).

Software & Drivers: The parent equipment needs to have software or firmware with drivers present on it. These components allow it to interpret the communications information it receives back from devices such as trip units. Device drivers need to be present for each device type, based on a device’s “Register Map.”

Network (wiring, cabling, equipment): Currently, power monitoring systems utilize wired networks. Low voltage trip units commonly use RS-485 networks (twisted-shielded pair cable, daisy-chained from device to device).

Wired network of power monitoring system diagram | Monitoring Power Distribution Systems


Device Configuration: In most applications, each device must be programmed with its own unique address. This is true for RS-485 Modbus communications. In addition to the address, each device usually has some configuration settings, like baud rate.

Diagram showing location and baud rate | Monitoring Power Distribution Systems | National Field Services

Incorporating the above items and constructing a power monitoring system can be involved and can often require specialty staff or communications experts. But not always…

Power Monitoring System Example

As devices become more technically advanced, power monitoring systems are becoming more complex. However, products, application and integration can still be simple!

Here is one example using Utility Relay Company’s AC-PRO-II® and/or AC-PRO® trip units and Smart 1-Line™ touchscreen HMI solution. Note that this system architecture leverages the circuit breaker trip units, and other than Smart 1-Line™, no additional products are needed.

System Components & Devices:
• AC-PRO-II® and/or AC-PRO® trip units (AC-PRO-II® with VDM recommended)
• Smart 1-Line™ touchscreen HMI

System Requirements & Installation Items:
• RS-485 network (twisted-shielded pair daisy-chained from trip unit to trip unit)
• Set each trip unit with a unique address
• Configure Smart 1-Line™ diagram and settings (takes hours, not days or weeks)
• Wall-mount in electrical room or control room

Smart 1-Line™ Product Features & Capabilities:
• Field configurable electronic “smart” one-line diagram
• Designed specifically for AC-PRO-II® and AC-PRO®
• Industrial touchscreen PC with no moving parts
• Metering, waveforms, graphics and more
• Software and drivers are pre-installed
• Wall-mount in electrical room or control room


Smart 1-Line™ Screens & Information:
Some of the information shown on the main screen one-line diagram:
• System overview
• Breaker positions
• Metered values
• Alarms & status

Tapping a breaker symbol provides the following information:
• Waveforms
• Trip history
• Settings
• Real-time readings
• Time-current curves
• Remote breaker trip
• Breaker test data  



Power monitoring systems can offer many benefits. Establishing these systems often requires special equipment and specialty staff. However, once the requirements and product options are understood, incorporating a power monitoring system at your facility or your customer’s facility might not be as challenging or expensive as you think.

About Utility Relay Company

Utility Relay Company is a leading designer, manufacturer and marketer of microcontroller based, true-RMS solid-state trip units and trip unit retrofit conversion kits for AC and DC power circuit breakers. Utility Relay Company’s focus is providing high quality, reliable and versatile trip unit kits for low voltage circuit breakers.

For more information about Utility Relay Company and their products, including AC-PRO-II® and Smart 1-Line®, visit


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