Connecting a current transformer: wiring diagram and steps
A current transformer has two sides to connect: on the primary side it sits on the conductor, on the secondary side two leads run from S1 and S2 to the meter. Four rules decide whether the measurement is correct and safe – mounting direction, secondary wiring, earthing at one point and short-circuiting before the secondary circuit is ever opened.
The wiring diagram shows a 250/5 A current transformer on a meter. The steps below explain what matters and link to suitable transformers and the VA calculator.
Step 1: Mounting direction: P1 towards the supply
On the primary side every current transformer is marked P1 and P2 (formerly K and L). Mount it so that P1 faces the supply and P2 faces the load – the current then flows from P1 to P2.
For an ammeter reading the direction does not matter. For energy meters, power meters and transducers it determines the sign: a transformer mounted the wrong way round makes consumption appear as export.
Step 2: Connect S1 and S2 to the meter
The secondary terminals S1 and S2 (formerly k and l) are connected to the current path of the meter – on an energy meter, to the current input of the associated phase. In three-phase systems each phase gets its own transformer; transformer, current path and voltage path must belong to the same phase.
Length and cross-section of the secondary leads determine the burden. With 5 A secondary current, 5 m one-way lead length and 2.5 mm² copper, the leads consume 1.78 VA; with 1 A secondary current only 0.07 VA. The VA calculator adds the meters’ own consumption and suggests the right rated output.
Step 3: Earth the secondary circuit at exactly one point
If the secondary circuit is earthed, it is earthed at one point only – at S1 or at S2, as specified by the grid operator, the technical connection conditions or the design. A second earth forms a loop through which compensating currents flow and distort the measurement.
Step 4: Short-circuit before opening
While primary current flows, the secondary circuit must never be open: the transformer keeps driving its current, and dangerously high voltages appear at open terminals. Before a meter is disconnected or replaced, S1 and S2 are short-circuited.
Traditionally a CT test terminal does this. On the plug-on CTs ALO and ALO-E, the KALO split-core CTs and the WALO wound primary CTs the short-circuit bridge is in the rotary cover: turn the cover and the transformer is short-circuited – no test terminal needed. Protection CTs have double secondary terminals for this.
Step 5: Remove the short circuit and check the reading
Only once the meter is connected is the short circuit removed: turn the cover back or open the test terminal. Set the transformer ratio on the meter, for example 250/5 A – with 200 A in the conductor, 4 A flow on the secondary side and the meter shows 200 A.
If an energy meter shows negative power on one phase, P1/P2 or S1/S2 of that transformer are swapped or current and voltage paths are assigned to different phases.
Typical Applications
- Energy meters with CT connection
- Analogue and digital ammeters
- Transducers and energy monitoring
- Protection relays on protection CTs
Recommended Products
FAQ
Where are S1 and S2 connected?
S1 and S2 are the secondary terminals of the current transformer. They run to the current path of the meter – on an energy meter, to the current input of the phase the transformer sits on.
What do P1, P2, S1 and S2 mean?
P1 and P2 mark the primary side, S1 and S2 the secondary side. P1 faces the supply. Older transformers are marked K, L (primary) and k, l (secondary).
Does a current transformer have to be earthed?
Whether the secondary circuit is earthed is specified by the grid operator, connection conditions or design. If it is earthed, then at exactly one point – at S1 or S2, never both.
What happens if the secondary circuit is open?
With primary current flowing, dangerously high voltages appear at the open terminals. The transformer is therefore short-circuited before every disconnection – on ALO, KALO and WALO via the short-circuit bridge in the rotary cover.
Why does my energy meter show negative values?
Usually a transformer is mounted the wrong way round (P1/P2), S1 and S2 are swapped, or current and voltage paths belong to different phases.