Understanding features and options about CTs: What to know before you buy.

How do I pick the right item?

When learning about CTs. There are two important considerations, above all else, to look at when buying a current sensor:

1. What kind of input signal are you trying to monitor?
2. What kind of output signal do you want the sensor to have?

If you get either of these two elements wrong, you’re going to be disappointed when you install and use the current sensor. Other considerations, such as size, style, etc. are secondary.

The most popular current sensors we sell expect an AC current input and output an AC voltage. Although there are many different meters and each has its own input requirements, most power monitoring meters expect 333 mV AC current. If your meter expects a higher AC voltage, we can customize our product to meet your need. Simply add one of our “accessories” to your item, such as the 0-5V Output Option. Ensure that the number of these that you order matches the number of current sensors you are ordering as well.

What is a current transformer and how does it work?

This article explains the principles about CTs. The basics of operation and how a current transformer differs from a current sensor with a voltage output.

A current transformer is a device that “transforms” or “steps down” the current input on the “primary winding” to an alternating current of equal proportion on its “secondary winding,” or output. In this way current transformers can convert a potentially dangerous current to one that is more manageable and easier to work with. Because the output current is proportional to the input, it’s ideal for power monitoring, controlling devices, etc. because we can know what the actual current is on the primary conductor by measuring the corresponding current on the secondary output.

True current transformers are passive devices, meaning that they do not require external power. Rather, they use electromagnetic principles to function. More specifically, they typically contain a laminated core of low-loss magnetic material. Next, a wire is wound around the laminated core. The number of windings, or “turns,” is inversely proportional to the current desired on the secondary winding, as expressed by this equation:

(Secondary Current) = (Primary Current) * (Number of turns on the primary conductor / Number of turns on the secondary conductor). We abbreviate this as Is = Ip * (Np/Ns)

In most situations with power monitoring current transformers the number of turns on the primary conductor = 1, that is to say, the conductor is simply passed through the center hole of the transformer, so in this situation we get:

Is=Ip*(1/Ns),orIs=Ip/Ns.

The most common “true” current transformer used for power monitoring and power controls has a 5 Amp AC current output, but 1 Amp AC currents also exist. Having said this, many current sensors in use today use a large number of windings, resulting in a very low current output. Many industries are preferring this type of output because it’s easier to work with. Instead, what they often do is add a “burden” resistor to the secondary winding to create voltage. Voltage is defined by this equation:

Voltage = Current * Resistance, abbreviated V = I * R

Using this formula, let’s come up with a hypothetical current sensor. Let’s say that we want to produce 333mV when 1000 Amps are “sensed” on the primary conductor, which in our scenario will be a bus bar passing through the center. If the current sensor has 7500 turns, we would expect 1000/7500 Amps, or 133 mA of current if no burden resistor existed. But in our case we want 333 mV of output, so we can divide 333 mV / 133 mA (or .333 V/ .133 A) and we find that the needed burden resistor should be 2.5 Ohms. Once burdened in this way, we can ignore the amperage output (it’s pretty small after all) and consider this a “voltage output” device. Because the current output is alternating current (AC) the output voltage is also alternating, abbreviated Vac.

Generally current transformers with a 1A or 5A output should not be left open-circuited or operated without a load when current flows on the primary conductor. Instead, one should short-circuit the secondary terminals to avoid the risk of shock. A device called a shorting block exists for this very purpose. When installing a 1A or 5A current transformer one needs to first short the secondary terminals (typically via the shorting block mentioned), and after the secondary terminals are connected to their load the short-circuit (shorting block) is removed.

However, all of J&D’s split-core CTs has an over-voltage protection circuit inside this is an advantage we have over some competitors the steps mentioned above are not necessary.

Current sensors with a 333 mV output don’t have this risk because the current output is extremely low.

Current sensors that change the output type are called current transducers. The hypothetical current sensor described earlier would be most accurately called a current transducer, yet they often simply get called current transformers because they operate using the same basic principles as a current transformer.

What is a current transducer and what types do you sell?

This article explains how a current transducer differs from a current transformer.

A current transducer modifies the input on a primary conductor to a different type of signal on the secondary conductor. In the strictest sense, even AC voltage output devices are current transducers.

So why should you care? It’s critical that you understand the input signal type and the output signal type when purchasing. The majority of our returns come from a misunderstanding of this concept than from anything else.

Transducers can operate passively (without external power) or operate actively with external power.

We sell the following transducers:

1. AC Amp in > AC Voltage out
2. AC Amp in > AC Amp out
3. AC Amp in > DC 4-20mA out
4. AC Amp in > DC Voltage out
5. DC Amp in > DC Voltage out
6. DC Amp in > DC Amp out
7. AC Voltage in > AC Voltage out
8. AC Voltage in > AC Amp out
9. DC Voltage in > DC Voltage out
10. DC Voltage in > DC Amp out

#1, 2, 3, 4, 7, 8, 9 can be self powered or (passive). With #5, 6 and #10 the excitation power typically comes from the DC loop that is created with the meter. In addition to being passively powered, #3, 5, 6, 7, 8, 10 also work with DC loop power or 24V DC.

Understanding amperage ratings and burden resistors

This article explains amperage ratings and how they to use them when selecting a device

This article explains amperage ratings and how they to use them when selecting a device.

Amperage ratings are used to guarantee a certain output at a given input. For example, a JM21N-100 will output 333 mV when 100A is “sensed” on the primary conductor. A JM21N-150 is an identical product, except for the burden resistor inside the two devices. The JM21N-150 will output 333 mV at 150A (and 222 mV at 100A).

Many customers are initially surprised to find out that a 3000A rated device can cost the same as a 250A device. This occurs when the only difference in the devices is the burden resistor, e.g. the JRFS-1800-250 versus the JRFS-1800-3000.

With most current transformers the product is guaranteed to be accurate with a given range of the rated current. Often this range is from 1% to 120% although this varies slightly depending on the product type. Using a JM21N-100 as an example, the product will be IEC61869-2 (IEC60044-1) 0.5S accurate measuring from 1A up to 120A.

Understanding this range is particularly helpful when purchasing a CT where you expect to measure at lower ranges most of the time, but will occasionally see higher amperage inputs. For example, say you are measuring current up to 500A, but you are most concerned with a much lower amperage, let’s say in the 50A range. One could purchase a JS36SL-600, but the 1% guarantee of accuracy cannot be reached at 50A. Instead the best choice would be a JS36SL 400. It can measure down to 40A with 1% accuracy and yet will still be 1% accurate up to 520A.

Another concern of customers is safety. Often the question is whether the device will be safe when operating at a higher amperage than the rated amperage. Safety and the amperage rating of a device are two different concerns. The products are of course guaranteed to be safe when operating at the rated amperage, but just because a device is rated at 100A does not mean it cannot be safe at a higher amperage (although it typically won’t be as accurate). Instead, what determines safety is the internal wiring of the device, i.e. how thick are the wires winding the core. Devices that are UL listed have gone through testing that determines this upper limit. If the same-model device is sold with an amperage rating higher than the one you are purchasing, you can be assured that the device is safe up to that higher amperage. For example, a JSC-02-050 will be safe up to 800A because a JSC-02-800 is also sold.

Once an inspector found an JM21N-100 installed on a 200A power panel line and gave the installer a bunch of grief. It took some time to convince the inspector that the JM21N-100 was safe up to 200A. Although the JM21N-100 probably wouldn’t product 666 mV at 200A, it certainly wasn’t going to melt or anything of that nature. In this case the installer knew that the customer would want to measure accurate down at 10A (and seldom if ever at 200A), so it was more important to have accuracy on the low end rather than the high end.