Electrical installations have protective devices with a certain circuit breaker capacity. Their purpose is to deal with these currents. However, if they can’t cope safely, that can lead to damages and fires. Therefore, you need to measure prospective fault current (PFC) and calculate its values at the source and other relevant points in the setup. Why? This guide describes everything you should know about PFC tests. That includes precautions, the testing process, and other details.
Table of Contents
The Basics of Prospective Fault Current
Caption: Electricity – abstract illustration
Prospective fault current is the overcurrent that occurs when something goes wrong in an electrical installation. For example, a short circuit might happen and cause PFC to flow. Alternatively, there could be an earth fault in the wiring, which leads to PFC.
We can consider two different values for prospective fault current:
- Prospective short-circuit current. It occurs between two Line conductors on three-phase supplies. Additionally, it could happen between Neutral and live conductors in a single-phase installation.
- Prospective earth fault current. Unlike the previous case, this situation occurs when the current flow between the Earth and Live protective conductors.
First, you analyze these values. Then, you can identify the highest value and consider it PFC.
What Is the Maximum That Prospective Fault Current Can Reach?
Caption: An electrical tester and a solar battery
The actual value is relative to the installation. However, the maximum fault current is always at the location with the lowest resistance value of the conductors. That’s why we conduct a PFC test at the point where the setup originates.
It’s usually the main switch, but always check the metering equipment provided by the distributor. There could be switchgear with a direct connection to it, so make sure to perform the PFC test in those areas, too. If you measure the current at other points, it won’t reach its maximum value.
Why You Should Do a Prospective Fault Current Test
Prospective fault current test will show if you need to adjust your protective gear. For example, you can have switchgear with a 6000A rating. These are common in domestic premises. But if your property is near a supply sub-station, the external loop impedance could be 0.03Ohms. Therefore, a 230V supply might generate 7,666A of PFC in your domestic installation. That implies you need switchgear with a higher rating. You can confirm this with the tests.
As per regulation 612.11 of BS 7671, the possible fault; prospective fault either under short circuit or earth fault conditions will be determined for every relevant point of the installation.
Here is an overview of the benefits of the PFC testing method:
- This is live testing, so you’ll have precise results.
- You don’t need advanced calculations, which make testing simple.
- It’ll improve safety for employees and any visitors.
- Your insurance premiums will be smaller, which means you’ll save money in the process.
- It helps to avoid serious gear problems, which minimizes potential downtime. It also decreases the cost of repairs of equipment made.
What Precautions to Take Before a PFC Test?
Caption: Electrician at work
Fault conditions are strongest at the installation’s origin point. That’s where you’ll do the testing, so caution is necessary. All tests are live, so make each move carefully.
First, check if all the connections are as usual. Abnormal conditions might give inaccurate results. It should be like the gear is ready for normal operation, including earth, main, and circuit protective bonding conductors. You should wear personal protective equipment (PPE). The person doing the test should be comfortable with the process.
Don’t forget to apply other precautions to stay safe from risks, such as conducting a ground fault test procedure.
How to Measure Prospective Fault Current and Its Maximum
Caption: Circuit testing
You need to have the right electric testing equipment and trained staff. First, make sure you have a PFC tester adequate for your setup. Next, the person doing the test should have the right training. Additionally, don’t forget the safety gear.
The process has two sequences:
- Prospective short circuit current test
- Prospective earth fault current test
Testing Prospective Short Circuit Fault Current
Caption: Short circuit illustration
- You begin by using the tester. Choose the PFC function or any other adequate for this test. The main switch should be off, but the electricity should be active.
- Check the main switch’s incoming side and find test leads. Connect them to the Neutral and Line terminals.
- Hit the action button to start the test. You’ll get the value in kA, so make sure to write it down.
If you have 3-phase supplies, you’ll need multiple tests. Depending on the test meter, you might need additional settings. Some units require connecting the third lead to the Neutral. You can find all details in the manual.
After testing a phase, you’ll receive a value. Make sure to double it to get a better idea of a maximum PFC. Using a higher protective rating leads to safer gear and reduces the risk of issues. If a short circuit occurs, it’s necessary to conduct an analysis of why it happened.
Testing Prospective Earth Fault Current
- Take the test meter and choose the PFC option. You should keep the supply active but deactivate the main switch.
- You can connect the leads of your tester. Connect them to the Line and Neutral conductors and the Earth.
- Hit the button to start the test. Check the reading and write it down (it will be in kA).
You should double the value of all readings in three-phase installations.
Find the Highest Value
Do you have all the readings from the tests? It’s now time to choose the highest value. That will be your final PFC value, so add it to the Electrical Installation Certificate. Next, check the protective gear and its breaking capacity. You need it to be higher than the PFC value. That way, you’ll secure the necessary protection rating of your setup.
If your supply types have three phases, you’ll find maximum PFC between line conductors. You’ll need a 400V capacity of the test meter to do the test. If you don’t have it, you can calculate the PFC between lines.
The experts agree about the way to calculate the maximum current in a three-phase supply. You need to double the value of a single-phase supply. First, you measure the value of prospective electrical fault current in a single phase. If it’s 4kA, you need to double it. That means the maximum PFC will be 8kA.
PFC tests are crucial to the safety of electrical installations in facilities. At Cloom, we have expertise and experience in electrical wiring. If you need any help with your cable assemblies, don’t hesitate to contact us!