Electrical steel can show different core loss values depending on how it is tested. Two of the most widely used methods are the Epstein frame and the Single Sheet Tester (SST).

Both methods are standardized for evaluating the magnetic properties of electrical steel, but they use different specimen formats and magnetic circuits. As a result, their core loss values may differ even when the same material is tested.

For engineers and buyers, the more practical question is whether the test method and conditions match the specification used for material evaluation or acceptance.

Epstein Frame vs SST at a Glance

Factor	Epstein Frame	Single Sheet Tester (SST)
Sample Format	Multiple strips assembled into a square frame	One larger sheet specimen
Magnetic Circuit	Formed through overlapping specimen strips	Closed with external magnetic yokes
Common Standard	IEC 60404-2	IEC 60404-3
Sample Preparation	Requires multiple cut strips	Requires fewer cuts
Typical Use	Standardized material characterization and specification testing	Standardized sheet-level magnetic characterization and production or laboratory testing
Main Measurement Considerations	Strip preparation, joints and corner regions	Yoke properties, contact conditions and field distribution

Results from the two methods should be interpreted within their respective test standards rather than treated as directly equivalent.

Why Can Core Loss Results Be Different?

Much of the difference between Epstein and SST results comes from how each method forms the magnetic circuit around the specimen.

In an Epstein frame, several electrical steel strips form a square magnetic path. Because the strips overlap at the corners, the joints influence how magnetic flux passes through the specimen, creating local variations in the magnetic field.

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Epstein frame with electrical steel strips for core loss measurement

Sample preparation also matters. Cutting the steel into narrow Epstein strips can introduce edge stress, which may alter the local magnetic domain structure and affect measured core loss.

The Single Sheet Tester uses a larger sheet specimen positioned between magnetic yokes. This reduces the number of cut edges and avoids the overlapping corner joints found in the Epstein frame, but it introduces a different set of measurement factors.

Single sheet tester measuring electrical steel with magnetic yokes

The external yokes form part of the magnetic circuit, so yoke properties, specimen contact, and small air gaps can influence field distribution. The effective magnetic path is also defined differently from the Epstein method.

The gap between the two methods can also change with polarization or flux density. A difference measured at one test point may not remain the same at another.

Does SST Always Give Higher Core Loss?

SST values can be higher than Epstein values for the same electrical steel, but the difference is not fixed. Comparative measurements have shown that the gap can change with polarization and material type, particularly for oriented and HiB electrical steels.

The size and even the direction of the difference can depend on grade, polarization, specimen preparation, rolling direction, and the measurement system. Grain-oriented steel may show a different relationship from non-oriented electrical steel because its magnetic properties are strongly directional.

A fixed conversion such as:

SST value = Epstein value × 1.05

should therefore not be treated as a general engineering rule. A factor observed for one grade or test setup may not apply to another material or operating point.

For low-loss grades, even a small difference in W/kg can affect grade comparison. Thickness is another factor that can influence electrical steel loss, especially as operating frequency increases; see our guide to electrical steel lamination thickness and core loss.

How Should Buyers Compare Core Loss Data?

For material acceptance, use the test method required by the material specification, customer standard, or purchase agreement. When comparing suppliers, first check whether the reported values were measured on the same basis.

At minimum, the following information should be reviewed:

Test method: Epstein frame or SST
Applicable standard: such as IEC 60404-2 or IEC 60404-3
Frequency: for example, 50 Hz or 60 Hz
Flux density or polarization: such as 1.5 T or 1.7 T
Specimen direction: rolling direction, transverse direction, or another specified orientation
Sample condition: including cutting, stress-relief treatment or annealing where applicable

Consider two suppliers quoting similar electrical steel:

Supplier	Reported Core Loss	Test Method
Supplier A	P1.7/50 = 0.90 W/kg	Epstein Frame
Supplier B	P1.7/50 = 0.94 W/kg	Single Sheet Tester

Supplier A appears to offer the lower-loss material, but the two values should not be ranked until the test conditions are aligned. Check the test standard, specimen direction, sample condition, frequency, and magnetic polarization first.

For purchasing, define the test method together with the core loss limit. A requirement such as P1.7/50 is more meaningful when the applicable standard and specimen conditions are also stated.

Conclusion

Epstein and SST results can differ because the specimen geometry, magnetic circuit, and test conditions are different. For purchasing or material acceptance, compare data on the same test basis and specify the required method where core loss is part of the contract.

HWHG supplies grain-oriented electrical steel and non-grain-oriented electrical steel for transformer, motor, generator, and other electrical applications. If your specification includes requirements for grade, thickness, core loss, or testing standards, our team can review them before quotation.

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HWHG electrical steel coils for transformer, motor and generator applications

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