Tag: Ct and pt

What is a CT Meter?

  By








A CT meter is simply a meter that is used in conjunction with instrument transformers known as current transformers. These are also known as CTs. In electrical metering, meters are divided into two types. There are self-contained meters. In addition there are transformer-rated meters. Transformer-rated meters are also known as CT meters.

What are the characteristics of a CT meter?

The characteristics of a CT meter include its ratings. CT meters now are generally rated at 20 amps. This means that the current coils of the meter are capable of handling 20 amps. You may think that this is low. But, remember that CT meters are used with CTs. Also remember that CTs have outputs determined by their ratios. They are rated with an output on the secondary side of 5 amps. If you remember, when using the rating factor of a CT it is possible for the CT to put out 20 amps.

CT meters also have voltage ratings. Many of the meters now are multi-range. This means that the meter can sense the incoming voltage and adjust its calculations based on the incoming voltage. Most meters now show the voltage on the display. Before digital meters, one had to be careful to make sure that the meter with the correct voltage rating was chosen.

What types of CT meters are there?

When talking about meter types what we are really referring to are the meter forms. Meter form numbers are used to designate what type of meter we have. These meter form numbers help us to decide which meter to use in which installation based on Blondel’s Theorem.

The normal transformer-rated meter form numbers are as follows:

Form 3s

Form 4s

Form 5s

Form 9s

Now remember that these are the most common. There are more.








Where will you find CT meters?

CT meters are installed on services that are too large for self-contained services. This normally means services that are larger than 200 amps. Although there are now self-contained 320 amp meters as well as 400 amp bolt in meters. CT meters are also used whenever PTs, potential transformers, are used to step down the voltage.

Large residences, commercial and industrial buildings, hospitals and schools are all examples of where you will find a CT meter installed.








Testing a Form 9s transformer rated installation

  By









The form 9s meter is perhaps one of the most popular meter forms used in metering. It is a versatile meter that can be used to meter either a 120/208 three phase four wire wye service or a 277/480 three phase four wire wye service. Can the form 9s be used to meter other services? Yes but I am only going to cover these two here. When testing a form 9s transformer rated installation you will be looking at a few different things. You will check the meter, the wiring, the CT’s and/or PT’s, and the voltage as well as the transformer.

First a disclaimer. If you are not a meter tech or a qualified person then you do not need to attempt anything that is written below in regard to the form 9s. It can kill you if you do not know what you are doing.

When we check the form 9s meter in a transformer rated installation there are a couple of things that we will look at and take note of. First we want to check the meter number. I know this sounds simple but you want to make sure that you are in the right place. Now write down the readings of the meter. There should be a simple kwh reading as well as a kw reading. It is also possible that your utility uses more than these such as kva. You may also be required to download the information from the meter as well. If the meter is a solid state meter you want to make sure that there are no diagnostic codes in the meter. If there are you will need to check them and find out what the problem is.

Solid state meters have come a long way. If you do not have a piece of test equipment that is capable of showing you the vector diagram of your form 9s meter you can logon to the meter and view the vector diagram there. The vector diagram can alert you to things that you may not be able to see right away. Vector diagrams can also let you know if you have any wires crossed as well as the amplitude of the current and voltage on all three phases. Using vector diagrams you can also view all of your phase angles.

We are still talking about the meter here. If you have an electro-mechanical form 9s meter then the procedure is a little different. There is nothing to logon to. The meter may have lights for each voltage phase. If so, you want to make sure that they are all lit up. Next, you want to do what is know as an element check. You will also do this with the solid state meter but you will have to leave all of the voltage switches in as the display likely comes off of A or C phase. To do the element check you will open all of the switches with the exception of the neutral switch. Then you close the voltage switch and the current switches for the phase your will be working on. You are doing this to ensure that each element in the meter causes the disk to rotate in a forward direction. Remember that the disk rotates in a counter-clockwise rotation.








After checking the meter you will want to check the wiring of the form 9s. Give the wiring a good visual check. Look at all of the terminations that you can see and make sure that the color code is correct. If you suspect overheating and think the wire needs to be replaced do so as soon as possible. Wires that are out in the sun tend to crack after several years so you want to make note of this. If the wiring needs to be replaced make a work order to do so. Look at the wiring on the CT’s and PT’s. Does everything look good? Improper wiring is one of the biggest causes of lost revenue with a form 9s metering installation. If the wiring looks good, let’s move on to the CT’s and PT’s.

I am not going to go in depth in this post about testing the CT’s and PT’s, that is for a later date. What I want to tell you here is to visually inspect the CT’s. If you can see the nameplate then that is great. Make sure that the multiplier on the meter matches whatever the CT ratio says that it should be. After that, make sure that the multiplier on the meter is what the multiplier is in the computer system.

This is one of the other big mistakes that you will find with the form 9s. It is so easy to input a multiplier incorrectly into the system. That is why you need to check it for every transformer rated meter that you test. After you verify the CT ratio on the nameplate use test equipment to test the CT. This may be a CT burden test or an admittance test. You can also do a ratio test.

After the CT’s you will want to check the PT’s. Verify the wiring and make sure that you do have voltage on all three phases. The PT’s are so much easier than the CT’s. On the subject of PT’s we always want to check the voltage and make sure that it is the correct voltage for the service.

Next we want to check the transformer. Look at the transformer and make sure that it is not leaking oil anywhere. Also, visually inspect all of the secondary connections that you can see and make sure there is no overheating. If you have an infrared camera now is a good time to use it and check the connections for hot spots. Be very careful when working around transformers as it is possible to have the primary connections very close to the secondary. Make sure that you are wearing all of your PPE before doing any of the aforementioned work.

You will want to check the KVA of the transformer and compare it against the KW reading of the meter which is called demand. If you cannot get the power factor from the meter either because it is a mechanical meter or you do not have a way with another piece of equipment you can use 80% for the power factor.

To convert KW to KVA you will divide the KW by the power factor. This will give you KVA. Let’s do the calculation. Let’s say that you have a form 9s meter that has a KW reading of 0.8 with a multiplier of 80. First, multiply 0.8 x 80 = 64 KW. Then if you cannot get a power factor reading assume 80%. 64/.80 = 80 KVA. Now that you have the KVA check to make sure that it is in the limits of the transformer. If it is you are good. If not get with engineering to make a change.

There it is. That is how to can check a form 9s metering installation. As always, be careful and always wear your PPE.








What are test switches for?

  By ,









Every now and then in the electric metering field you will run across a transformer rated metering installation that does not have a test switch. Is this a good thing or a bad thing? Most CT rated installations require that test switches be installed. These switches can be used for a few different things. To test the meter, to shunt the CT’s, to safely remove the CT rated meter from service and they can also be used to check the voltage and the amperage on the service without actually having to open an enclosure or go into a fence.

Why do certain installations not utilize test switches? The answer here would most likely be cost. The meter bases or CT rated installations that you will typically find without test switches are form 3s and form 4s meter bases. Form 3s and form 4s meters are many times found on large residences and sometimes large temporary services such as construction services or school trailers that are not thought to be in service very long. The cost of installing the test switch along with the cost of the larger meter base to hold the test switch is often times a deterrent. Also, in the case of residences one could argue that they just do not use enough power to justify putting a large meter base and test switch on the wall because the meter will be changed out when all of the form 2s meters are changed on their neighbors homes.

Installing a transformer rated service without a test switch can be a bad thing. One thing that you have to remember is that when you pull the meter in a CT rated service and you do not use a test switch is that you are opening the circuit of the secondary side of the CT. This leads to a build up of voltage on this circuit which is dangerous to metering personnel. The proper procedure without a test switch would then be to shunt the secondary side of the CT before pulling the meter.

Test switches can be used to test the meter. With different types of test equipment they can be used to test the meter in service using the load that is available at the customer’s site. This can be a good test to show exactly how the meter is metering the service under the load that is currently on the service. You can also test the meter using a phantom load while it is still in the meter base using the test switches.








Test switches are also used to test the CT’s in the service. You can use various different types of test equipment to test the CT’s. You can test the burden on the CT circuit as well as determine how many amps are on the CT circuit as well.

To remove the meter from service you need to shunt the CT. There is a switch that does this for you. Shunt the CT out and you can safely remove the meter from service. You can also use the test switches to remove all voltage from the meter as well before removing the meter from service and before installing the meter in service.

With the new regulations regarding arc-flash hazards and safety, many utilities have adopted safety policies that no longer allow their personnel to work inside energized cabinets, pad mount transformers or other enclosures if the service is too large or if the voltage is too high. This is yet another thing that the test switch can be used for. It can be used to check and make sure that the customer is getting the proper voltage. You can also check the rotation in the meter base at the test switch as well.

All in all, it is a good practice to install test switches in all of your CT rated metering installations. They will allow you to test the meter in service, test the CT’s in service as well as allow you to check voltage and rotation. Test switches also allow you to safely install and remove meters from service by isolating the blocks of the meter base from current and voltage.