Stromwandler24

What does a current transformer do?

Current transformers are special transformers that proportionally convert high currents into directly measurable smaller currents. Their design and physical operating principle provide safe galvanic isolation between the primary and measurement circuit.

Connect current transformers correctly

The primary winding terminals are marked with the capital letters K and L or P1 and P2. Polarity must be set so that energy flow runs from K to L or from P1 to P2. The secondary winding terminals are marked with k and l or s1 and s2. Polarity must be set so that the energy flow direction runs from K to L.

Swapping terminals S1 and S2 leads to incorrect measurement results.

Avoid open-circuit operation of current transformers

A current transformer operated with an open secondary side induces very high secondary voltages. These can reach several kilovolts and therefore pose a major risk to people and equipment.

Therefore, the secondary circuit must never be opened while current is flowing in the primary circuit. Open-circuit operation must be avoided and transformers must be shorted at the secondary terminals when connecting or replacing measuring devices.

Do current transformers need to be grounded?

At low voltage, usually no; from a system voltage of 3.6 kV, yes. Grounding provides safety in the event of a winding fault between primary and secondary side. It is also common to ground low-voltage transformers. Grounding can be done at terminal S1(k1) or S2(k2). Transformers should always be grounded on the same side.

Correct burden sizing of a current transformer

When sizing the transformer burden, consider the connected measuring devices as well as cable length and cross-section. The required transformer burden results from the burden of devices plus cable. A transformer with too little VA burden (overburdening) cannot maintain its accuracy class and measurement error increases.

Types of current transformers