Current Transformer Standards And What They Control

The Standards In Use Across This Range

• IEC 61869-2 — the current standard for current transformers, replacing IEC 60044-1
• IEC 60044-1 — the previous instrument transformer standard, still quoted widely
• BS 7626 — the British implementation, seen on UK and Commonwealth specifications
• VDE 0414 — the German instrument transformer standard
• DIN 43703 — the European DC shunt footprint and rating standard
• ANSI/IEEE C57.13 — the North American instrument transformer standard
• GB 1208 — the Chinese national current transformer standard
• IEC 61010-1 — safety requirements applied to the shunt range
• UL94 V-0 — the flammability rating carried by the moulded cases

What Each One Actually Controls

The IEC and BS standards define the accuracy classes, the burden series, the short time thermal current and the dielectric tests. DIN 43703 defines the shunt footprint and the standard millivolt drops. ANSI/IEEE C57.13 defines a different class structure, which is why a North American specification cannot simply be read across to an IEC one.

The Figures On A Rating Plate

• Ratio, for example 400/5 A
• Accuracy class and rated burden, for example 0.5, 10 VA
• Rated voltage and insulation level, for example 0.72/3 kV
• Short time thermal current, usually as a multiple such as Ith = 60 × In
• Rated dynamic current, typically 2.5 × Ith
• Rated frequency and rated security factor, for example FS < 5

Why FS Matters

The rated security factor limits how much secondary current a measuring transformer will pass under fault conditions. A low FS means the core saturates early, which protects the connected instrument. It is the number that makes a measuring core unsuitable as a protection core.