1. Assumptions Limitations
Adaptive Ground Fault Circuit Interrupter
End Product Description
Abstract
A ground fault circuit interrupter, or GFCI, is a device used to protect against electric shock if someone should come into contact with an energized electric wire while also in contact with the earth/ground. A leakage current between the energized wire to ground may also exist, and vary with time.
One of the functions of the existing GFCI module manufactured by the Square D Company is to trip, or open, a circuit in the event that the leakage current should exceed 4-6mA. However, since the leakage increases gradually with time, this results in the circuit tripping unnecessarily. The solution proposed by this project is to provide a control to adjust the set threshold for gradual increases in the leakage current.
The expected result of this project will be an AGFCI that will monitor and adjust the threshold current that will trip, or open, a circuit breaker in a power system with the use of a micro controller. The AGFCI will be designed to fit a typical residential or commercial circuit breaker rated at 120 volts, in accordance with the specifications as provided by the Square D Company.
Existing GFCI
Design Requirements
Introduction
Design Objectives
The objective of this project is to design a modification to Square D’s standard GFCI. This addition will include the use of a micro-controller to adjust the threshold point of a GFCI, depending on the leakage current detected by the circuit.
Design Constraints
The first of constraint is that the completed circuit will ultimately need to fit inside a typical breaker. For the time being, there is no specific size limit on the size of the chipboard in the final product. The second is the size of the program that will need to be stored in the microprocessor, as well as the speed of the microprocessor itself. These last couple of constraints will not be extremely important considerations as another micro-controller with the proper specifications can be chosen. The micro-controller will also require a power supply. Since the current GFCI has a power supply already, consideration will be taken to add a second, or use the existing supply.
Measurable Milestones Functional Requirements
General Background
This project will produce an AGFCI that will control the operation of an electrical breaker/switch. The resulting device will fit into a typical electrical breaker designed for use in residential and/or commercial buildings.
Technical Problem
The solution devised to the existing problem will consist of a micro-controller to control the adjustment of the threshold value in the GFCI. The device will take information about the leakage current and act on that information based on a custom design program. This program will be written in C language, converted to assembly, then downloaded to the micro-controller.
Operating Environment
The product will be expected to operate normally within –35o to 80o C. It will be also be able to withstand damp conditions. Also, the entire module will be enclosed inside a breaker box, and must be able to operate inside an enclosed environment.
Intended Users and Uses
The AGFCI is intended for use in breaker for residential and commercial installation. It will be used specifically to protect people from electric shock, and appliances from damage due to short circuits. The Square D Company will be the sole manufacturer of this product, since they will own the result.
Assumptions Limitations
Range of Operations 2.0 – 20.0 milliamps
High Noise Immunity
Operates at 120 Volts
Response Time
30 msec when leakage is less than 10 mA
15 msec when leakage is greater than 10 mA
Threshold Increase Rate of 0.5 mA in 60 sec
Threshold Decrease Rate of 0.5 mA in 10 sec
Problem Definition
Study Current GFCI
Choose Solution Method
Define Program Parameters
Completion of Program
Completion of Prototype
Protoype Functional
Leakage current increases according to a definite function
Leakage current will have a value large enough to detect
Financial budget
Time
Available micro-controller technology
Technical Approach
The first consideration was how the leakage current would be detected, but a current sensor from the original design will be used. The sensor’s output will then be examined by a micro controller. The micro controller will then determine if the circuit will trip. The next consideration was what the most appropriate coding language would be. The decision here was to use the assembly language provided with the micro controller. The team is fairly familiar with this type of language and any higher-level language would need to be converted. The choice was made to use any controller that would have the memory capability to carry all of the program code on board. The next step is to determine the method that will trip the breaker.
Budgets
Personnel
Financial
Total Estimated Time
400 Hours
Total Estimated Budget
$350
Testing Approach
Team Members
All testing will be done to meet the functional requirements as provided by the Square D Company. The code’s modules will need to be tested separately and as a whole program. Also, in addition to functionality tests, the speed of the program will be a testing consideration, since the response to a potentially life-threatening situation is a concern. This speed testing will mostly involve testing for the most efficient ways to achieve the desired output. The prototype will be tested by passing a varying differential current and observing the results.
Members
Paulus – Team Lead
Mike Lin
Pat Roehl
Emeka Anyanwu
Advisors
V. V. Sastry
James Triska
Glenn Hillesland
Acknowledgements
Breaker Load Center
The Square D Company
Henry Zylstra