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.