Milbank is pleased to announce high-amperage Current Transformer CT cabinet additions to our product portfolio. These bussed cabinets range from to A. We continue to work with our reps and utility contacts to obtain more approvals nationwide. Installation Options: Overhead or underground entrance, underground entrance and bottom exit and pad-mount or wall-mount. Features: Three-point latching handle, NEMA 3R rated for indoor or outdoor applications, aluminum piano hinged door swall-mount brackets provided, CT supports included for window style CTs.
Applications: Used where higher-amperage power is needed: Large residential, multi-tenant, commercial buildings.
Applications: Used where a long-run junction box or service termination is needed. Applications: Used where higher-amperage power is needed in conjunction with a disconnect. CT Cabinets.
CT Cabinet Interconnection
Installation Options: Overhead or underground entrance, underground entrance and bottom exit and pad-mount or wall-mount Features: Three-point latching handle, NEMA 3R rated for indoor or outdoor applications, aluminum piano hinged door swall-mount brackets provided, CT supports included for window style CTs SCCR: 50k up to A units, 65k A—A units Applications: Used where higher-amperage power is needed: Large residential, multi-tenant, commercial buildings.
Installation Options: Overhead or underground entrance, underground entrance and bottom exit and pad-mount or wall-mount SCCR: 50k up to A units, 65k A—A units Applications: Used where a long-run junction box or service termination is needed. Combo Disconnect Cabinets.Forums New posts Search forums.
Location Orem, UT. There is a home that I am going to be installing on, and I cannot for the life of me, figure out is this is possible The home has an existing CT cabinet which is connected to the main serivce panel. One thing that I'm wondering, if it's okay to actually backfeed via a supply-side connection, or even a load-side connection in these cases. The energy that is provided by the Solar system won't interfere with the existing CT rings, or if a new NET meter was installed, will it still monitor customer consumption vs production?
Carultch Senior Member. Location Massachusetts. The NEC would have no issue with it, but the utility likely wouldn't allow it assuming it is a utility service meter's CT cabinet.
By contrast, if the utility has nothing to do with the meter and its CT cabinet in question which is uncommonthere would be no issue.
Some utilities might be more flexible, but most will not allow branching or tapping of customer circuits within the enclosure they seal. If you were allowed to do this, it would be a supply-side interconnection, using either a vacant terminal on the load side of the CT cabinet, or a tap connector to intercept all load-side conductors of each phase.
If a net meter is installed, it would still measure consumption minus productionas long as you connect both circuits on the load side terminals.Most meters used on homes or farms are self-contained.
All electric energy used passes through the meter. These meters are designed to be used on services under amperes. Current transformers are contained internally.
Current Transformers for Metering
When services are more than amperes, transformer-rated meters are used. As the name implies, these types of meters use current transformers CTs to measure the current flow, or total power being consumed.
The information is recorded by the meter. In donut-type CTs, there are two conductors, or windings. The primary winding is the line conductor that passes through the center of the CT. The secondary winding is the multiple turns of magnet wire around the core. The CT transforms the primary current of the line conductor to a smaller, more easily managed current that is carried to the meter which is directly proportional to the primary current.
This current is inversely proportional to the number of secondary turns of wire around the iron core. The Burden rating B is the impedance of the circuit connected to the secondary winding. This impedance is the total opposing effect to the flow of current in an AC circuit. The Burden rating is the maximum amount of impedance before exceeding the minimum accuracy limits. The current ratio difference between the actual current primary and the metered current secondary results in what is normally referred to as the multiplier.
The correction factor is the factor by which the reading of a watthour meter must be multiplied to correct for the effects of the error ratio and phase angle of the CT. If your Heat Rate Program requires the metering of storage heat, baseboard heat, dual fuel, or any other electric heat, low-grade current transformers will simply not do. Our metering-grade current transformers are made from premium grade laminated silica steel cores and are manufactured to meet IEEE C Current Transformers for Metering There are two types of electric meters: self-contained direct drive and transformer rated.
We have the solution! High-Quality Metering Current Transformers If your Heat Rate Program requires the metering of storage heat, baseboard heat, dual fuel, or any other electric heat, low-grade current transformers will simply not do. Available Ratios Accuracy BO.When alternating current travels through an electrical conductor, like cable or bus, it develops a magnetic field at right angles to the flow of current. Photo: Wikimedia. The principal function of a current transformer is to produce a manageable level of voltage and current, proportional to the current flowing through its primary winding, for the operation of measuring or protective devices.
In its most basic form, a CT consists of a laminated steel core, a secondary winding around the core, and insulating material surrounding the windings. If this current is passed through the primary winding of a CT, the iron core inside becomes magnetized which then induces a voltage in the secondary coils. If the secondary circuit is closed, a current proportional to the CT ratio will flow through the secondary.
CTs are typically wired to a terminal block where shorting screws can be installed to tie isolated points together. It is important that a CT always have a burden or load connected when not in use, otherwise a dangerously high secondary voltage can develop across the secondary terminals. There are four typical types of current transformers: window, bushing, bar, and wound. The primary winding can consist merely of the primary current conductor passing once through an aperture in the current transformer core window- or bar-typeor it may consist of two or more turns wound on the core together with the secondary winding wound type.
Window and Bar Type CTs are the most common current transformers found in the field. Photo Credit: ABB. Window current transformers are constructed with no primary winding and can be of solid or split core design.
These CTs are installed around a conductor and are the most common CT type found in the field. Installation of solid core window CTs require the primary conductor to be disconnected. Split core window CTs can be installed without first disconnecting the primary conductor and are commonly used in power monitoring and metering applications.
Zero Sequence CTs are a type of window CT that is typically used for ground fault sensing on a circuit by summing the current on all conductors simultaneously. Under normal operation, these currents will vectorally sum to zero. When a ground fault occurs, since a portion of the current will go to ground and not return on the other phases or neutral, the CT will see this imbalance and send a secondary current signal to a relay.
Zero sequence CT's eliminate the need to use multiple window CTs that have their outputs summed together by instead using a single CT that surrounds all of the conductors.#Current_Transformer -- Metering CT -- -- Protective CT -- -- CT Burden --
Bar-type current transformers operate on the same principle as Window CTs but have a permanent bar installed as a primary conductor. Bar types are available with higher insulation levels and are usually bolted directly to the current caring device. Bushing current transformers are basically window CTs that are specially designed to fit around a high-voltage bushing. Usually these CTs cannot be accessed directly, and their nameplates are found on the transformer or circuit-breaker control cabinet.
Wound current transformers have a primary and secondary winding just like a normal transformer. These CTs are rare and are usually used at very low ratios and currents, typically in CT secondary circuits to compensate for low currents, to match different CT ratios in summing applications, or to isolate different CT circuits.
These type of current transformers have very high burdensand special attention to the source CT burden should be applied when wound CTs are used. The CT voltage class determines the maximum voltage the CT may come in direct contact with. For example, a V window CT cannot be installed on or around a bare V conductor, however a V window CT can be installed around a V cable, if the CT is installed around the insulated portion of the cable and the insulation is rated correctly.
The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of is rated for primary amps at full load and will produce 5 amps of secondary current when amps flow through the primary.Many people want to know what is a current transformer and potential transformer. One thing that I want to note also is that CT rated meters are not only used as a secondary electric meter, they are also used as a primary electric meter as well.
CT rated meters are also typically demand meters as well. Transformer-rated services run in parallel with the service. This also depends on the policies and procedures of the utility. For example, some utilities require anything over v to be transformer-rated. While other utilities do not.
Click here for a power efficiency guide. I recommend against this practice for the safety of the meter tech or lineman who may need to install or remove these meters from service. As stated before they serve to step down high current to a safe a manageable level. When amps are flowing through the primary side of the CT, 5 amps are coming out of the secondary terminals.
The most important things to note on the nameplate are the ratio and the rating factor. The ratio will be printed in large letters on the side of the CT.
Typical ratios are,and so on. Again, what this means is that when the stated value of amps is flowing through the primary side of the CT, 5 amps is flowing through the secondary side. The rating factor is used when determining what size CT to use in a particular installation. What this means is that the manufacturer says the CT is accurate beyond its nameplate value. For example, a CT that has a rating factor of 4 will accurately measure a service up to amps.
So, if that particular service were to have amps on it, there would 20 amps coming out of the secondary side of the CT and in the meter base. Meaning that we want a CT to be sized so that the amps flowing through the primary side has as close to amps as possible. When the core of the CT is fully saturated it is the most accurate.It is essential to inspect and test current transformers and their connected instruments at regular intervals. Photo Credit: ABB.
Current transformers play an important role in the monitoring and protection of electrical power systems. CT's are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments. The importance of instrument transformer tests is often underestimated. Current transformers for metering purposes must have a high degree of accuracy to ensure precise billing while those used for protection must react quickly and correctly in the event of a fault.
Risks such as confusing instrument transformers for metering and protection, or mixing up connections can be reduced significantly by testing before initial use.
At the same time, electrical changes in a CT, caused for example by aging insulation, can be identified at an early stage. For these reasons and more, it is essential to inspect and calibrate current transformers and their connected instruments at regular intervals.
There are 6 electrical tests that should be performed on CT's to ensure accuracy and optimal service reliability:. CT ratio is described as the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of will produce 5 amps of secondary current when amps flow through the primary.
If the primary current changes, the secondary current output will change accordingly. For example, if amps flow through a amp rated primary the secondary current output will be 2. Unlike the voltage or power transformerthe current transformer consists of only one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped around the core or just a conductor or bus bar placed through a central hole.
A CT ratio test can be performed by injecting a primary current and measuring the current output, or by injecting a secondary voltage and measuring the induced primary voltage. Photo: TestGuy. The ratio test is conducted to prove that the ratio of the CT is as specified, and to verify the ratio is correct at different taps of a multi tap CT. The turn's ratio is equivalent to the voltage ratio of potential transformers and can be expressed as follows:. N2 and N1 are number of turns of secondary and primary windings V2 and V1 are the secondary and primary side voltage readings.
Ratio tests are performed by applying a suitable voltage below saturation to the secondary of the CT under test while the primary side voltage is measured to calculate the turns ratio from the expression above. Applying a saturation voltage will result in readings that won't be accurate. The polarity of a CT is determined by the direction in which the coils are wound around the transformer core clockwise or counterclockwise and by how the leads are brought out of the CT case.
All current transformers are subtractive polarity and should have the following designations to visually identify the direction of current flow:. A CT under test is assumed to have correct polarity if instantaneous current direction for primary and secondary current is opposite to each other.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. The characteristics of a CT meter include its ratings. CT meters now are generally rated at 20 amps.
What is a CT Meter?
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.
Metering Current Transformers
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. 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. CT meters are installed on services that are too large for self-contained services.
This normally means services that are larger than amps. Although there are now self-contained amp meters as well as 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. What is a CT Meter? What are the characteristics of a CT meter? What types of CT meters are there?