We sometimes use an old self build 1:1 isolation transformer (150VA) in our company to isolate devices under test.
It is equipped with a primary 500mA fuse (slow) in series with a on/off switch. However, due to inrush currents when the transformer is first energized, the fuse tends to blow from time to time. I am aware that the inrush current can already be pretty large for a transformer of that size.

I would also like to use an even smaller fuse, since 500mA is still pretty much from a shock perspective and most DUTs require far less current.

Would it be an option to just move the fuse to the secondary side of the transformer or would you advise against it for safety reasons? If a secondary fuse is fine, would you advise leaving the switch on the primary side?

  • \$\begingroup\$ Precisely what does the fuse protect? \$\endgroup\$
    – Andy aka
    Jul 16, 2014 at 10:07
  • \$\begingroup\$ @Andyaka: Well, from what I understand, it would prevent the transformer from heating up if there is a short on the secondary side and potentially protect a defective DUT from sinking too much current. The only safety you get from isolation is that you potentially can touch one wire and be safe and the current limiting isn't it? \$\endgroup\$
    – Rev
    Jul 16, 2014 at 10:13

2 Answers 2


If you move the fuse from the primary side you have no protection in case the insulation of the primary coil of the transformer fails.

Maybe the best solution is to have a fuse on each side: e.g. 2.5 A on primary and 250mA on secondary.

BTW a fuse is not enough to protect against electrical shock. Something lika a RCCB (inserted on the secodary side) is needed for an effective protection.

  • \$\begingroup\$ Fusing both sides and primary with higher current fuse now seems so obvious. However I thought a RCCB wouldn't be of much use with an isolated transformer? To get shocked, you have to touch both secondary wires anyway, so a RCCB wouldn't detect any leakage current would it? \$\endgroup\$
    – Rev
    Jul 16, 2014 at 9:34
  • \$\begingroup\$ @Rev1.0: I'm not sure about that. If the capacitance of all the wiring of the DC insulated secondary side to ground is large enough, some AC current flow is possible that could pose a risk. \$\endgroup\$
    – Curd
    Jul 16, 2014 at 13:09
  • \$\begingroup\$ True, but the common RCCB is designed to trip on a leakage current of >30 mA. \$\endgroup\$
    – Rev
    Jul 16, 2014 at 13:49

An old question, but hopefully I have a still-relevant comment to do with people-protection rather than DUT protection.

Some background. In days of yore it was, I believe, a Regulatory requirement to fuse both sides of the secondary (and, to a person performing checks outside the transformer case, it always appears as though the two fuses were in series, and until I checked by looking inside the case and also checking old Regulations, I wondered at the point of "two fuses the same value" in "series").

Those two secondary fuses not only protected the secondary of the transformer against overload, but they also provided (admittedly minimal) protection for the user of the transformer against the chance of a short circuit between primary and secondary windings, thus completely negating the primary people-protective reason for the transformer.

So, to answer your question, by all means do use protection on the secondary side, but make it two fuses, one on each secondary wire, to protect against that admittedly rare primary to secondary short circuit.

Or to be more prudent as you have been advised for a slightly different reason, use two RCCBs in the same configuration.


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