Introduction
Polyphase transformer-rated watt-hour meters make up the bulk of the industrial metering systems installed in the field. They are also installed in a variety of commercial applications as well. In this post, I want to show you how you can perform a load check on polyphase transformer-rated metering applications. Learning how to perform a load check on polyphase tranformer-rated metering installations will help you verify that everything is installed and working correctly. Let’s get started.
Tools
To perform a load check on a polyphase transformer-rated metering installations, you only need a few essential tools. There are also a couple of tools that will help you, but are not absolutely necessary.
- Pliers: The first tool you are going to need is a good set of pliers. These will help you cut the seal and get into the meter base.
- Screwdriver: The next tool you may need is a good screwdriver. You may need a screwdriver to help you open the meter base and you may need a good screwdriver to help you open up the CT cabinet. Normally a slotted screwdriver does the trick, but sometimes CT cabinets are secured with Philips head screws.
- Nut Driver: Sometimes you need a nut driver to get into the meter base or CT cabinet.
- Voltmeter: You will need a voltmeter to check the voltage and current on the site. I recommend a good clamp meter so you can easily check the amps on the site.
- Optional: A laptop is an optional tool to help you check transformer-rated metering installations. If you have a laptop and vendor specific software along with the appropriate optical probe, you can verify that your metering installation is wired correctly using a computer and vector diagrams.
This is just a small list of tools that will help when you are performing a load check on a transformer-rated metering installation. If you want to see more tools that I recommend, visit my Tools for Meter Techs page.
PPE
Sorry, but I have to mention PPE. If you are going to perform load check on a transformer-rated metering installation, you are going to be working on live circuits. Anytime you work on a live circuit you must wear all the appropriate PPE. This includes hard hats, safety glasses, rubber gloves rated for the voltage at the installation, boots, and FR clothing with the appropriate calorie rating for the available arc flash potential. Working on live circuits is dangerous, and you should not perform any work on a live circuit without proper training. This post in no way qualifies anyone to perform this work. You must be qualified by your employer. I take no responsibility for the actions you may take while working on live circuits based on the information in this post.
Step 1
The first step is to verify that you are at the correct location. This involves looking at your paper work and comparing the address with the meter number and making sure they match. You are also going to look at the form number of the meter and verify it is the correct form number for the installation.
Step 2
Now that you have verified the address and you know you are in the right place you are going to go ahead and remove the cover from the meter based. In a transformer-rated metering installation, there should be a test switch. I am going to be covering a form 9s metering installation here, but the basics apply for other transformer-rated metering forms as well.
Step 3
Inside the meter base there is a test switch. We are going to start by identifying the voltage switches and the current, and current return switches. Get out your voltmeter and go ahead and check the voltage switches for the proper voltage. Let’s go ahead and say that we are checking a 120/208v 4 wire wye service with a form 9s meter and 200:5 CTs. There should be 3 voltage switches on the test switch. A common test switch setup for form 9s meters is to use a 10 pole test switch. There will be 3 voltage switches, 3 current switches, 3 current return switches, and one neutral switch.
When you check voltage between all three voltage switches, you should get about 208v. When you check each switch to ground, you should get about 120v. This normally runs a little high so it is not uncommon to see something around 215v and 125v.
You can now go ahead and clamp your clamp meter around each of the current wires. This will tell you how much current is on the service. Since you know what the multiplier is, you can multiply the amps on your clamp meter by the multiplier to tell you how many amps you should get on the primary side of the CT. Now take your clamp meter and put it around the primary wire in the CT cabinet or transformer. You should be able to do the math to verify that the CT ratio is correct and that the multiplier is correct.
Step 4
Now that you have used your voltmeter to determine which switches are the voltage and current switches and used you clamp meter to verify the multiplier and CT ratio, you now want to verify the phasing. This works better with an old electromechanical meter but you can do it with solid state meters as well. A phase voltage has to match up with A phase current for a form 9s metering installation. You want to open all the switches and then close the neutral, A phase voltage, and A phase current and current return switches. If A phase voltage and current are properly wired, the meter will run forwards. If not, you have a problem and you will need to trace the wires to see what is going on. Repeat this for the other two phases.
Step 5
Once you have done all the verification steps above, you are done with performing a load check on a transformer-rated metering installation. Now all you need to do is close up the meter base and CT cabinet.
Conclusion
I hope this helps understand what all goes into performing a load check on a transformer-rated metering installation.
Further Resources
If you want more information on some of the topics above, be sure to visit: