Magnetically soft ferrites Mn-Zn and Ni-Zn exhibit relatively high electrical resistivity, demonstrating almost insulating properties. Two typical materials were tested for their Comparative Tracking Index (CTI) performance, with surprisingly high values of voltage withstand for the Ni-Zn ferrite. However, they cannot be classified as insulating materials from the viewpoint of creepage distance required for safety with hazardous voltages.

Magnetically soft ferrites (ferrous oxides) do not exhibit the same high electrical conductivity as solid metals. The high resistivity of ferrites allows their operation at much higher frequencies than it is the case for cores made from alloyed laminations (including thin foils such as nanocrystalline ribbons), due to suppression of eddy currents.

The general chemical formula of spinel ferrites is MeFe₂O₄ where Me usually represents one or more divalent transition metals such as Zn, Mn, or Ni, (but also Co, Cu, Mg, Cd).¹⁾ There are two major soft ferrite groups important in industry, typically referred to as Mn-Zn and Ni-Zn ²⁾.

For lower frequency applications the Mn-Zn ferrites offer higher relative permeability (typically up to several thousands) and are widely used for example in switch-mode power supplies for electromagnetic energy conversion. Ferrites are ferrimagnetic so their magnetic saturation point is lower than for iron-based ferromagnets. However, for frequencies above 100 kHz the saturation point is typically less relevant, because in power-conversion applications the designs become loss-limited, rather than saturation-limited³⁾

The other group is based on Ni-Zn and exhibits much higher resistivity and lower permeability (typically in the range of tens to several hundreds).⁴⁾⁵⁾ These ferrites are used in higher frequencies applications, with their operation extending even to the GHz range.⁶⁾

Ferrites are already oxidised so they do not undergo the same corrosion processes (as Fe-based metal alloys), and hence they can operate even without any protective surface coating. However, despite their high resistivity they cannot be treated as electrical insulators, as shown below by the experimental verification with the CTI test method (described at the end of this article). The Mn-Zn ferrites fail immediately (thus not attaining even the worst Material Group IIIb). The Ni-Zn ferrites perform much better, but are still unsuitable as electrical insulators, and for safety purposes should be treated as conductors.

Caption ^{Copyright © by XXX, YYY, CC-BY-4.0}

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

The Mn-Zn samples were arbitrarily chosen as core halves of the ETD49 type, material 3C95 ⁷⁾. The nominal initial magnetic permeability is ur = 3000 +/- 20 %, and maximum permeability of approximately 5000. Magnetic saturation is 0.53 T at 25 °C. The size of each core half was 50 mm x 17 mm x 7 mm (of flat tested part) [ http://ferroxcube.home.pl/prod/assets/etd49.pdf ]

The electrical resistivity is specified by the manufacturer as 5 Ωm [Ferrox], which is more than 8 orders of magnitude higher than that of copper. However, resistivity of common insulation materials such as PVC typically exceeds 1010 Ωm, which represents a completely different class of resistivity. [https://mycableengineering.com/knowledge-base/pvc-insulation ] [https://pvc.org/about-pvc/pvcs-physical-properties/electrical-insulation-characteristics/ ]
The CTI test was carried out at 400 V (AC rms). Immediately upon application of the test voltage there was a significant amount of current conducted through the body of the ferrite, and a glow-like activity occurred after just 1 second of test (Fig. 1a), followed by a fracture of the core and flash over between the test electrodes after 2 sec Fig. 1b), with the current exceeding the allowed 1 A limit. 

It is most likely that the fracture occurred from mechanical stress due to rapid thermal expansion caused by the heating from the conduction current. All the tested samples failed in the same way (Fig. 2), before the drips were applied.

The current was tripped almost immediately, faster than the current meter could stabilise. All I know is that it was > 1 A at 400 V which is what is written in the log. I have just run the test again on one of the broken ferrite pieces. Again, the current is tripped too quickly for a current reading to be read. Testing again and gradually raising the voltage from zero: at AC 80 Vrms the current reads 70 mA, at 90 V the current reads 110 mA. Somewhere between 90 V and 100 V the current increases to > 1 A and the trip operates.

The Ni-Zn samples were arbitrarily chosen as the flat cores 7427247 used for EMC suppression in flat ribbon cables. [Wurth, https://www.we-online.com/components/products/datasheet/7427247.pdf ] Typical initial permeability of this material (4 W 60) is ur = 620. The size of each core was 76 x 28 x 5 mm (Fig. 3)[Wurth]. The electrical resistivity is typically above 105 Ωm [https://www.magnet-tech.com/core/nizn/characteristics.htm] [https://www.acme.com.my/acme2/c_acme_company_magnectic3.htm], which is much higher than those of the Mn-Zn ferrites, but still significantly lower than the insulating materials. The samples were preconditioned for over 24 hours at 50 % relative humidity, at 23 °C, before testing (as required by the CTI test standard), each sample being removed from the preconditioning chamber as it was needed. The first sample at 600 V (AC rms) between the probes tripped the 1 A overcurrent breaker at the third drip, thus failing this level of test. The second sample was tested at 400 V between the probes and it passed – the overcurrent breaker did not trip and there was no persistent arcing or flame after 50 drips. The third sample at 500 V and it failed, tripping the 1 A overcurrent breaker at the 18th drip. This Ni-Zn ferrite material has proven itself to be a conductor at the tested voltage and should be considered as such when designing isolation barriers, despite the apparent pass of the 400 V limit for one sample.

In this test the same shaped sample was equipped with two copper-tape electrodes, separated by 40 mm (Fig. 4), which is equivalent to basic clearance up to 16 kV DC (11.3 kV AC). The tester was capable of applying up to 19.3 kV, but when the voltage was slowly increased the ferrite conducted at 8 kV (AC rms). There was no visible tracking along the surface but there was some scintillation in the centres of each copper tape – the current flowed through the ferrite, warming it considerably. In the after-test picture, the marks left by the scintillation can be seen on the edge centres of the tape.

Comparative Tracking Index is a method (and a parameter) used for assessing the relative immunity of insulating materials to developing conductive surface tracks (tracking), which can bridge the insulating gap between the test electrodes. The method for testing is defined for instance by IEC 60112:2020 Method for the determination of the proof and the comparative tracking indices of solid insulating materials or ASTM D3638-21e1 Standard Test Method for Comparative Tracking Index of Electrical Insulating Materials. The test provides an accelerated testing of ageing due to voltage stress, humidity, and contamination [ASTM]. During the test, organic materials become carbonised, thus creating surface “electrical treeing”, with branches growing randomly, but generally propagating between the electrodes. https://webstore.iec.ch/en/publication/32739 https://www.astm.org/d3638-21e01.html

The method involves applying two chisel-shaped electrodes on a surface of the material to be tested, spaced by 4 mm, applying voltage up to 600 V between the electrodes, and applying 50 drips of ammonium chloride (NH4Cl) (0.1% by mass solution in deionised water), 30 s apart, so that they fall between the probes to initiate chemical degradation of the surface. There must be no persistent flame or arcing and the test sample must not conduct to open the 1 A overcurrent breaker. According to the IEC standard, the full procedure is to have five samples pass the test at the set voltage. If one fail, then a further five must be tested with no further failures. In other words, five-out-of-five must pass or nine-out-of-ten must pass to confirm the CTI at the set voltage.

CTI Material Groups as defined IEC 60601-1:2005: [standard] Comparative Tracking Index (CTI) Material Group 600 ≤ CTI I 400 ≤ CTI < 600 II 175 ≤ CTI < 400 IIIa 100 ≤ CTI < 175 IIIb

Larger value of CTI means that the material is less susceptible to surface tracking. The distance between the two points in an electrical circuit must be then designed so that an appropriate creepage distance is provided, for example as dictated by the standard IEC 61010.

The authors would like to thank Megger Instruments Ltd. for providing all resources required for performing experiments described in this article.

• Magnetically soft ferrites
• Creepage