CT RATED AND

One of the most common meters we sell in the lab is the AC Ammeter. They range from microamperes to kiloamperes where .000001 A is equal to 1 microamperes and 1000 A is equal to 1 kiloamperes. The most common of the AC Ammeters that we build in the lab is the CT rated AC Ammeter.  CT meaning "Current Transformer".  For the most part when a customer asks for a CT rated meter they may ask, " I need a CT rated meter and my CT is 100 to 5 (represented as 100:5).  So what does this mean and how do we associate it to the panel meter?  Remember in the last bulletin we discussed some of the terms for panel meters.  Two of these terms were "input" and "scale". For a CT rated meter of 100:5 we would provide to the customer a meter that has an "input" of 5 AC Amps and a "scale" of 0-100 AC Amps. Simple isn't it.  If I have a customer looking for a CT rated meter and his CT is 600 to 5 (600:5) then we provide him with a meter that has an "input" of 5 AC Amps and a "scale" of 0-600 AC Amps. In both examples above the only other information we would need is  "what size meter" 1 1/2", 2 1/2", 3 1/2", supply the customer with a meter.

           
In general when we look at CT rated meters we consider that the input to these meters is 5 AC Amps.  The scale depends on the "ratio" of the CT as in the two cases above of 0-100 AC Amps would be the scale and the ratio 100:5 or 20:1(same ratio as 100:5) and the 0-600 which is represented with a 0-600 AC Amp scale and ratio of 600:5 or 120:1.

           
Lets consider the two examples above, in order to measure 100 AC Amps or 600 AC Amps two things are required, a CT and a panel meter with a rating/scale appropriate to the CT ratio.

           
So why do we use a CT?  When we talk about panel meters we're talking about very delicate, small devices.  When we attempt to measure "a lot of current" (amperes) it requires that the device or panel meter be able to handle the amount of current  that we're trying to measure.  In layman's terms it takes a solid piece of copper about the size of the lead in a No. 2 pencil to handle 5 AC Amps.  To handle 50 AC Amps it takes a solid piece of copper almost the size of the whole pencil.  So you can see why the higher the current or amperes we are trying to measure the larger the requirement is for the wire to handle the current.  Now as we try to measure more and more current it's not practical to use such large wire in such a small space. So now when we need to measure a large amount of current or amps the industry standard would be to use a "CT" and a CT rated meter.

           
One common mistake is that a customer may have an application where they need to measure a large amount of current.   Let's say 200 AC amps.  A customer might suggest that they need a CT rated meter to measure 200 AC Amps but they want to be sure that the CT will handle the amount of current so they might say "they would also like to use a CT with a ratio of 250:5" instead of 200:5.  This in respect to panel meters will not work.  In order to have the correct combination to measure 200 AC Amps we must use a 200:5 CT.

           
Now I know you've already figured this out, that for every CT rated meter that we sell there is a good possibility to sell the "current transformer" to go along with each panel meter.   

A note on current transformers.  They are represented in a format of ratios which can mean that a ratio of 100:5 is the same as 20:1.  Sometimes a customer will tell you that his CT ratio is 120:1.  120:1 is the same as what?600:5.  For advanced applications there can be ways to "manipulate" the CT by winding the primary and/or secondary leads.  Some engineers can get very creative so you can count on being surprised once and a while.

           
Okay, primary and secondary on CT's.  Let's start off simple.  Let's take the basic "doughnut" transformers (Vendor code IX and a part number 5RL101 100:5).  We'll use the 100:5 CT as an example: If you want to picture this think of a doughnut with two wires coming out of it.  These two wires are about the size of the pencil lead we talked about earlier.  Too small to handle 100 amps, right.  So these two wires are the "secondary" wires of the CT.  Now if you've pictured the doughnut and the two small wires coming out are secondary where is the primary hook-up?  In this case a doughnut transformer is a simple way to "transform" 100 amps to 5 ACA. This is done by taking only one lead of the 100 Amps (of the circuit that we're going to measure, referred to as the "load") and put it through the center of the doughnut.  This is how the primary is hooked up to a doughnut transformer.  It's convenient because these large wire sometimes can't be interrupted and it's very economical especially when you get to even larger amounts of current.  Now we have a relatively large wire that is positioned through the center of this doughnut which is our "primary" hook-up and the small wires that come out of the CT (doughnut) are hooked up to the CT rated panel meter that has a input of 5 ACA and a scale of 0-100 ACA. So now when 100 amps passes through the wire and through the CT this will generate 5 ACA out of the secondary.  Five amps will go to the back of the meter and with 5 amps applied as the input the pointer moves to "full scale" which in this case is 100 AC Amps.  As the 100 amps of current varies up or down so does the amount of output on the secondary and so does the pointer on the meter.  With the same CT and same panel meter, when the load is 50 amps or half the 100, the output on the secondary is what? Half of the 5 amps which is 2.5 Amps.  Now with 2.5 amps as an input to the meter instead of reading 100 the meter reads half or what? Right, 50 amps.  Wow,  your math is getting better.

          
Another common type of AC Ammeter is a "self contained" AC Ammeter.  A self contained AC Ammeter is an ammeter that will accept exactly what the "scale" indicates.  Let's use 50 AC Amps. If a customer calls in and asks for a 0-50 AC Amp Ammeter, our first question is do you want a "self contained" ammeter or are you using a "CT"?  If the customer says, "I don't have a CT", this means he may need a "self contained" meter.  In this case he has no CT so he hooks his input directly to the back of the meter and the meter is capable of measuring 50 AC Amps. So...the input is 50 AC Amps and the "scale" is 0-50 AC Amps.  

So where's the dividing line between "when can a panel meter be self contained or when will it be a CT rated meter?  First always listen to the customer, if we're working from "just" a description, I would use 100 AC Amps as a guide line.  Anything below, we need to verify with the customer "self contained" or "CT rated"and anything over 100 may only be available as a CT rated meter.  All this varies between models and manufacturer's.  When we work from part numbers this job is a bit easier.  The key is knowing what questions to ask.  Hope this helps just a little.

There are always exceptions to all of these rules.  My intention is just to begin to give you a "basic" understanding of how these "things" work.

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