Current transformer connected meters do the heavy lifting in Australian commercial and industrial buildings. Where a load is too large for direct connected metering, external CTs measure the primary current and pass a reduced signal to the meter.
It is a simple idea. The compliance question sitting behind it is not. If that measurement is used to charge another party, the equipment falls under Australian trade measurement law. This is where the phrase NMI approved starts appearing in specifications, tender responses and product descriptions.
It is also where the phrase gets stretched further than it should be. Pattern approval applies to a defined instrument in a defined configuration. It does not travel with the meter into every CT arrangement an installer might build around it.
Key Points
NMI pattern approval covers a specific meter pattern described in a certificate of approval, not the current transformer (CT) that the meter is connected to.
A certificate states the approved energy meter current input, rated and maximum current, accuracy class.
Accuracy class belongs to the approved measuring instrument, so the meter is either Class 0.5S or Class 1.0 with a 5 A CT input, which does not take into account the CT accuracy where used.
Pattern approval and verification are separate legal steps, and a meter must be verified by a utility meter verifier before it is used for trade.
Trade measurement approval sits alongside National Electricity Rules, AEMO and network obligations rather than replacing any of them.
SATEC supplies pattern approved CT connected options under NMI 14/2/72 and NMI 14/2/80, covering both single circuit and multi circuit billing on Australian sites.
What Is A NMI Approved CT Meter?
A NMI approved CT meter is a transformer operated electricity meter covered by an Australian NMI pattern approval certificate.
External current transformers measure the primary current and deliver a reduced secondary signal to the meter. Transformer operated meters commonly use a 1 A or 5 A secondary. Other approved systems use dedicated current sensors designed and tested specifically for that meter, which is a different arrangement with different rules attached.
The legal trigger in Australia is narrower than most people assume. Electricity meters were exempt from trade measurement requirements for years while the framework was built. That exemption was lifted for meters installed on or after 1 January 2013 measuring less than 750 MWh per year. Tenant submeters and branch circuit billing meters sit comfortably inside that category.
Which Standard Applies Now
For a long time the answer was simply NMI M 6-1. That is no longer the full picture. NMI currently assesses active energy electricity meters against NMI M 6, NMI M 13 or NMI R 46 depending on the pathway taken. The verification procedure was rewritten in August 2024 to cover meters approved under all three documents.
When you read an older certificate you will usually see NMI M 6-1 named, because that was the applicable document at the time of approval.
What Pattern Approval Actually Covers
Approval applies to the configuration described in the certificate and its technical schedule.
For a CT connected meter, the certificate defines what NMI calls the field of operation. That typically includes the number of phases, number of wires, reference frequency, ambient temperature ranges, rated voltage, rated current, maximum current, meter constant and accuracy class. Features such as import and export measurement, time of use registers, data logging and communications are listed separately.
Where dedicated current sensors form part of the measuring system, the certificate names the specific sensor models permitted. An approval for a particular meter and sensor combination is a special testing arrangement under NMI.
What Happens When A Certificate Is Reviewed
Certificates are subject to review. A common worry is that an expiry or review date invalidates meters already in service. It does not. A meter can be installed and used for trade under an expired or cancelled certificate provided the certificate was valid at the time the meter was manufactured and NMI has not withdrawn it. Withdrawal is rare and reserved for serious problems.
Does The Approval Cover Every CT Ratio?
CT ratio is one of the areas where confidence outruns evidence.
A conventional approved CT meter accepts a standard secondary current such as 5 A. The primary ratio can then be selected to suit the circuit, subject to the meter design, the CT specification and the applicable metering rules. The meter must also be configured with the correct transformation ratio so recorded energy reflects the actual primary circuit.
Where a meter is approved with dedicated current sensors, the position is stricter again. Only the listed sensor models and their stated operating ranges form part of the approved measuring system.
Accuracy Belongs To The Whole Measuring Arrangement
Accuracy class is not a property of the meter electronics alone. Overall measurement is shaped by the current transformer, the CT burden, the secondary wiring, connection quality, phase association and configuration.
SATEC’s own certificate makes this point better than any general explanation could. Under NMI 14/2/72, the EM133-XM direct connected pattern and the 5 A CT operated variant both hold Class 0.5S. The same meter supplied with HACS current sensors holds Class 1 for each approved meter and sensor combination. Same instrument, different measuring system, different class. The maximum permissible errors applied at verification shift accordingly.
That is the whole argument in one example. Read the certificate rather than the accuracy figure on a datasheet, because the certificate tells you which arrangement the figure belongs to.
Pattern Approval Is Not Verification
Another frequent assumption is that an approved meter can go straight into service.
Pattern approval confirms the design is metrologically suitable for trade. Verification confirms that the individual instrument being placed into service performs within the permitted errors. Both are required. A meter that is not pattern approved cannot be verified at all, so there is no remedial path once it is on the wall.
Verification is carried out by a utility meter verifier using the National Instrument Test Procedures. The current documents are NITP 14.0 for general requirements together with NITP 14.2 for electricity meters, published in August 2024. Testing covers accuracy at prescribed current and power factor test points, running with no load and starting. Evidence takes the form of a verification mark or a certificate of verification identifying meters by serial number.
Then there is the part no certificate can help with. Ratio settings, polarity, phase allocation, secondary wiring and sealing all have to be right. An approved CT meter cannot compensate for a CT installed backwards.
What NMI Approval Does Not Replace
NMI approval is legal metrology. It is not approval of the electrical installation and it is not acceptance for every electricity market application.
Depending on how the meter is used, a project may also need to satisfy the National Electricity Rules, AEMO requirements, distribution network service provider requirements, embedded network obligations or state and territory regulations. For National Electricity Market connection points the complete metering installation attracts additional obligations. For private submetering and tenant billing the meter is one part of a longer compliance pathway.
Comparing Approved CT Connected Options
The table below compares three approved arrangements available in Australia.
| Attribute | EM133-XM-5A | EM133-XM-HACS | BFM136 |
|---|---|---|---|
| Certificate of approval | NMI 14/2/72 | NMI 14/2/72 | NMI 14/2/80 |
| Approved accuracy class | Class 0.5 | Class 1 for the meter and sensor combination | Class 1 for the meter and sensor combination |
| Current input | 5 A CT secondary | Named HACS solid core or split core sensors | Named HACS solid core or split core sensors |
| Rated and maximum current | In 5 A, Imax 10 A | Sensor dependent, 50 A to 1500 A rated | Sensor dependent, sensors up to 3000 A |
| Billing points per instrument | One three phase or three single phase | One three phase or three single phase | Up to 12 three phase, 18 two phase or 36 single phase |
| Installation note | Suits existing 5 A CT loops | Split core options avoid breaking conductors | Sensors can sit up to 200 metres from the meter |
| Typical application | Commercial and industrial single circuit billing | Retrofit metering in live switchboards | Multi tenancy switchboards with many billing points |
SATEC Options For NMI Approved CT Metering
Two pattern approved platforms cover most CT connected requirements on Australian sites.
The EM133-XM sits on certificate NMI 14/2/72. Its 5 A CT operated variant carries Class 0.5 for commercial, industrial, renewable energy and tenant billing circuits. Where a switchboard cannot be shut down or a conductor cannot be broken, the HACS sensor variants provide an approved retrofit route at Class 1, with solid core and split core sensors rated from 50 A upwards.
Where a single switchboard serves many tenancies, the BFM136 under NMI 14/2/80 changes the economics. A single instrument covers up to 12 three phase circuits, 18 two phase circuits or 36 single phase circuits using named HACS sensors, at Class 1 for each approved combination. Sensors can be mounted as far as 200 metres from the meter, which matters when the load is nowhere near the available panel space. Shorting diodes are built into the sensors, so CT shorting switches are not required.
Both platforms connect to Expertpower for remote data collection, reporting and billing management. That layer handles the commercial workflow. Legal measurement still rests on the approved and correctly verified meter installation underneath it, and that distinction is one we are careful never to blur.
Behind both certificates sits 50 plus years of energy measurement engineering, including the first DIN rail meter and the first multi circuit meter to achieve Australian pattern approval.
Check The Certificate, Not The Label
When specifying a CT connected meter for billing, ask for the certificate number and read the technical schedule. Check the model designation, the approved current input, the accuracy class attached to your specific arrangement, the permitted sensor models and the conditions of approval. Then confirm the instrument has been verified and installed with the correct CT configuration.
Certificates are public. Ten minutes of reading before purchase is cheaper than a billing dispute two years into a tenancy.
If you are specifying CT connected metering for an Australian site and want the certificate detail checked against your actual configuration, talk to the SATEC team. Send us the single line diagram and the circuit schedule, and we will tell you which approved arrangement fits and which one does not.
FAQs - NMI Approved CT Connected Meters
Does NMI approval mean I can use any CT with the meter?
These rules are subject to regulatory rules for off market, on market and system accuracy requirements.
Is a Class 0.5 meter still Class 0.5 when I add current sensors?
No. The class shown on a certificate belongs to the approved measuring arrangement, and a meter supplied with dedicated sensors may carry a different class for that combination. Always check the accuracy class stated against the variant you are buying.
Can I install an approved meter straight away?
Not for trade use. The individual instrument must be verified by a utility meter verifier under the National Instrument Test Procedures before it can be used to bill another party. Verification is evidenced by a verification mark or a certificate identifying the meter by serial number.
Does an expired certificate mean my installed meters are no longer compliant?
No. A meter can continue to be used for trade provided the certificate was valid when the meter was manufactured and NMI has not withdrawn it. Withdrawal is rare and applies only where a significant problem is identified.
