3 Reasons Why You Should Be Using PT’s

Why use PT’s? That is a good question and this post offers three reasons why you should be using PT’s in your transformer-rated services. First, let’s review what PT’s are. PT stands for potential transformer. Some people call them VT’s which stands for voltage transformer. The names can be used interchangeably. PT’s are used to step down the voltage in a transformer-rated service to a safer and more manageable level. If you want more information on PT’s then check out our page on CT’s and PT’s. That brings us to the three reasons why you should be using PT’s.


The biggest reason why you should be using PT’s in your transformer-rated services is safety. Safety is very critical for your metering personnel and lineman. So, how do PT’s increase safety? Remember that PT’s step down voltage from a higher voltage to a lower voltage. For instance, if you have a 3 wire 480v delta service you could use 4:1 PT’s to step the voltage down at the meter base to 120v. This is much safer for utility personnel to work with.

Anything above 240v should be using PT’s to increase the safety of metering personnel and lineman. This means that instead of allowing 480v 4 wire wye and 3 wire delta self-contained services you should consider using a CT cabinet with CT’s and PT’s no matter how small the service is. This can help prevent exposure to 480v.

Prevent Catastrophic Meter Failure By Using PT’s

Below you can see what happens when a self-contained 480v meter blows up. This particular service was not even

Form 12s 480v meter blows up

Form 12s 480v meter blows up

in use at the time. The building that this meter serves is vacant. This is an Itron Sentinel form 12s meter installed in a 480v 3 wire delta service. Things like this can and do happen. If however, this installation had been metered with instrument transformers and used a CT cabinet or even overhead metering this meter would probably have not blown up. It does need to be mentioned however that if something in the service caused this meter to blow up then most likely had PT’s been used, one of the PT’s would have blown as well.

I can hear what you are saying right now. Well, if the PT is going to blow to then what is the point? The point is that when a PT blows it is generally not a catastrophic failure like shown in the pictures. Sometimes when a PT goes bad it is even hard to tell from the outside that anything has happened. Yes, you will still have a PT to replace but the failure will not be catastrophic. Although not the case in the pictures, often times when a meter blows like this the meter base needs to be replaced as well. Also, when a meter fails like this it can create fires as well. Which brings us to our last reason why you should be using PT’s.

Reduce Customer Downtime By Using PT’s

When you have a meter failure like the one shown in the pictures what does that mean for your customers? That means that they are going to have some downtime while you make the necessary repairs. At a very minimum you are going to have to replace the meter. Many utilities now have policies that prevent their personnel from working on 480v services while they are still energized. This means that while any repairs are made power to your customers will remain off. This can be the case even in the event that you are using a transformer-rated service with PT’s. In the

Form 12s 480v meter failureevent of a meter failure on a self-contained service like the one shown above power will be turned off until a new meter can be set. If the meter base is burned up or if a fire is caused this can mean that an electrician will have to be hired by the customer and also that the fire department will come out and inspect as well. Sometimes this means that the customer will have to pass inspections before the service can be restored. This can be a few days sometimes. If however a PT fails, the power may only need to be off for 30 minutes while the bad PT is replaced and everyone can go on their merry way.


PT’s are an essential tool that can be used to increase the safety of utility personnel who work on metering installations. When used properly they can help prevent catastrophic failure of metering installations. On top of that when used, PT’s can reduce the downtime your customers experience by reducing the number of meter failures and by reducing the time it takes to make repairs when a failure does occur.

Form 12s Meter Wiring Diagram

The form 12s meter can be one of the more confusing meter forms out there. So, here I want to provide a form 12s meter wiring diagram in two different configurations. One is the 120/208 network service.  The other form 12s meter wiring diagram is the form 12s meter in a delta service.

Form 12s Meter Wiring Diagram Network Service

The form 12s meter can be used with the network service. The network service comes from taking two legs off of a three phase wye transformer and using the neutral. So, if you measure voltage from each phase to ground you should get 120v. Also, if you measure voltage from phase to phase you should get 208v.

This service is typically found in businesses that are located in a downtown area or businesses that are located in an industrial park. The reason for this is that many of these businesses do not require a full three phase service. So, the utility will pull two legs and a neutral off of the nearest wye transformer and call it a day.

Looking at the diagram you notice that there are two yellow terminals. These are either or terminals. Meaning that you connect the wire to one or the other in the meter socket. Also, some meter sockets do not come with this 5th terminal pre-installed. You have to purchase a 5th terminal separately and install it yourself.

With some form 2s meter sockets there are provisions to install a 5th terminal. The form 12s is also known as a self-contained meter. This means that the meter is installed in series with the load. Pulling the meter will also turn off the power.

Form 12s Meter Delta Wiring Diagram

For the form 12s delta wiring diagram we are going to talk about the service. This is the diagram for using a 12s with a delta service. Note that this is a three phase service. If you notice there is no difference in how it is actually wired. I used different colors here to denote the difference but it is actually physically wired the same way.

This could be a 120v, 240, or 480v delta service. My recommendation, as always, is against using a self-contained meter for voltages above 240v however. In the center of this diagram following the blue wire again this is an either or connection. You can connect the wire to the left center terminal or the bottom center terminal. Also note that the stab on the back of the meter is interchangeable.

Since this is a three phase service when measuring voltage across any of the three phases you should get the same voltage.

As always remember that the colors in the diagram are for illustration purposes only. You should always use the colors your utility uses.