Plug-on, cable split-core or busbar split-core current transformer: the types compared
The three common types differ mainly in one question: does the primary conductor have to be opened for installation? With the plug-on current transformer it does – the conductor passes through the closed window. Cable and busbar split-core transformers have a divisible core and are placed around the conductor; the circuit stays closed.
The closed core of the plug-on transformer provides the highest rated outputs in a small housing and is the only one of the three types also available calibrated (ALO-E). Cable split-core transformers are very compact and deliver little power – suited to digital instruments with short leads. Busbar split-core transformers open at the push of a button, click audibly into place and cover busbars up to 3000 A.
For both cable and busbar there is the KALO split-core transformer. All CELSA current transformers comply with DIN EN 61869.
The three types compared
| Plug-on current transformer | Cable split-core transformer | Busbar split-core transformer | |
|---|---|---|---|
| Installation | conductor through the window, circuit must be opened | place around the cable, secure with UV-resistant cable tie | open at the push of a button, place around the busbar, fix with clamping screws |
| Rated primary current | 1 A to 6000 A (small currents as wound CT) | 60 A to 1000 A | 60 A to 3000 A |
| Secondary | 1 A, 5 A, 333 mV | 1 A, 5 A, 333 mV | 1 A, 5 A |
| Accuracy classes | 0.5; 1; 3 – calibrated 0.2S; 0.2; 0.5S; 0.5 | 0.5; 1; 3 | 0.5; 1; 3 |
| Rated output | 1 VA to 30 VA | 0.2 VA to 1 VA | 1.25 VA to 60 VA |
| Secondary connection | screw terminals with rotatable, sealable cover | fixed, colour-coded leads | screw terminals |
| For billing (calibrated) | yes, as ALO-E / WALO-E | no | no |
| Typical use | new installations, control cabinet, main distribution | retrofit, energy monitoring, digital instruments | retrofit on busbars, high currents |
| CELSA series | ALO, WALO, ALO-E | TQ, TC | IAP, IAM, IAG |
Ranges across all versions of each type in the CELSA range; the product list shows which combination of current, class and output exists for each window size.
Calculators and related pages
Recommended Products
FAQ
What is the difference between plug-on and cable split-core transformers?
The plug-on transformer has a closed core; the conductor passes through the window, so the circuit must be opened. The cable split-core transformer has a divisible core and is placed around the existing cable – without shutdown. In return it delivers much less power (0.2 VA to 1 VA).
Do I have to shut down the installation for a plug-on transformer?
Yes, the primary conductor has to pass through the closed window. That is why plug-on transformers are mostly used in new installations. For retrofitting during operation there are cable and busbar split-core transformers.
When do I use a busbar split-core transformer?
When retrofitting an existing busbar that needs more power or higher currents than a cable split-core transformer offers: busbar split-core transformers are available from 60 A to 3000 A and with up to 60 VA.
Why do cable split-core transformers have so few VA?
The small divisible core only delivers a low rated output of 0.2 VA to 1 VA. That is enough for digital instruments and energy meters with low consumption and short leads. The cable loss calculator shows how long the lead may be.
Can I use split-core transformers for billing?
Not in the CELSA range: the calibrated types are the ALO-E plug-on and WALO-E wound current transformers. Cable and busbar split-core transformers suit operational metering and energy monitoring.
What does 333 mV secondary output mean?
Instead of a current, the transformer delivers a voltage of 333 mV at rated current. It is intended for energy meters and monitoring systems with voltage input and cannot be combined with devices for 1 A or 5 A.
Are split-core transformers less accurate?
CELSA also offers cable and busbar split-core transformers in class 0.5. What matters is that the burden suits the small rated output – otherwise the class is not met.