Current transformers
All CELSA current transformers in one list: plug-on CTs for new installations, split-core CTs for retrofits without disconnecting, calibrated CTs for billing, protection CTs for protection relays and summation CTs for several feeders. Made in Römerberg, Germany, to DIN EN 61869.
Not sure which transformer fits?
The assistant asks step by step about purpose, mounting and current and shows the matching articles.
Basics, types and selection criteria are explained in the current transformer guide →
Sub-groups
Products
Which current transformer fits?
| Type | Application | Primary current | Secondary | Classes |
|---|---|---|---|---|
| Plug-on CT ALO | new installation, switchgear, main distribution | up to 6000 A | 1 A, 5 A, 333 mV | 0.5 – 3 |
| Wound primary CT WALO | small currents with own primary winding | 1 A – 40 A | 1 A, 5 A, 333 mV | 0.5, 1 |
| Split-core CTs | retrofit without disconnecting, cable and busbar | 60 A – 3000 A | 1 A, 5 A, 333 mV | 0.5, 1, 3 |
| Calibrated CTs | billing to third parties | 1 A – 5000 A | 1 A, 5 A | 0.2S – 0.5 |
| Protection CTs | protection relays, overcurrent protection | 75 A – 4000 A | 1 A, 5 A | 5P5 – 10P10 |
| Summation CTs | several feeders on one meter | 2 – 9 inputs | 1 A, 5 A | 0.5, 1 |
The filters of the list narrow down ratio (e.g. 400/5 A) and window; every ratio has its own overview, such as current transformer 400/5 A.
Choosing a current transformer
Four values decide: the rated primary current (slightly above the highest operating current), the secondary current of the instrument (5 A, or 1 A for long measuring leads), the accuracy class (0.5 or better for billing, otherwise usually 1) and the burden in VA needed by instrument and lead together. The guide choosing a current transformer explains it step by step; the VA calculator works out the burden.
The type follows from the installation: if the conductor is free (new installation), the plug-on CT is the common and most economical solution. To retrofit a live installation, a split-core CT is placed around the cable or busbar. A comparison of all types is in current transformer types compared.
Frequently asked questions
What is a current transformer?
A current transformer maps a high primary current in a fixed ratio to a small secondary current – usually 5 A or 1 A – that meters, instruments and protection relays can handle safely. A 400/5 A transformer delivers 5 A to the instrument at 400 A in the conductor.
Which ratio do I need?
Choose the rated primary current just above the highest operating current of the conductor and the secondary current according to the instrument input. Every ratio has its own overview, e.g. current transformer 400/5 A or 100/5 A.
Does the circuit have to be interrupted for installation?
For a plug-on CT yes, because the conductor is passed through the window. Split-core CTs are opened and placed around the conductor – without interrupting the primary circuit.
Which current transformers are approved for billing?
Billing to third parties requires calibrated (conformity-assessed) current transformers together with a MID meter. CELSA makes the plug-on CTs ALO-E and the wound primary CTs WALO-E with PTB type approval for this.