I have a 110VAC input, 24 VAC output transformer that includes an Elsen WA128 fuse on the 110 v input side. I accidentally momentarily shorted the 24v. This blew the fuse. I want to replace the fuse with an equivalent, but I need to know if this is a slow or fast acting fuse.

This transformer supplies power to a skylight window controller. The window is opened and closed with a DC motor.

The description of where slow and fast acting fuses are used is confusing: fast for electronics, slow for motors. But this application does both.

Thank you for your responses. I admit that I was a bit sloppy - I said the transformer was 110 v, it is actually 120 v, I apologize. For further clarification, I have had this transformer in use for over 10 years without problems. It is rated at 55 VA. I accidentally touched the 24v lines together momentarily. A day or so later I realized that the skylight no longer worked. I found the problem to be the transformer. A VOM test showed that the secondary had continuity, but the primary winding was open. I drilled out the rivets and unwrapped some tape around the primary winding. I then found the Elsen fuse, connected in series with the primary winding. A VOM test shows continuity through the primary winding is good, but the fuse is open.

Although I am not certain about this, from the comments above, and other comments on the web, it appears that the Elsen fuse is both a thermal fuse, AND a current fuse. I am not sure about this, but it appears that the Elsen part is no longer made. I was able to find a possible replacement on eBay, but that is when I started thinking that a suitable replacement might be a better approach.

To my surprise, the momentary short circuit of the transformer also caused the skylight electronic circuit board to fail. This is a different problem, and also one that I have not yet solved, but a different subject.

The transformer is mounted in an open area in the basement, and the duty cycle is really really trivial. The skylight had not been used for several days before my momentary short circuit mistake. I seriously doubt that the fuse blew due to transformer heat over 128 deg C. I think it was due to a few milliseconds of secondary short circuit.

The part referenced above, thermal fuse NTE8070-8242, will pass up to 15 amps. I have not been able to find the current rating of the Elsen WA128.

My guess is that this transformer was NOT made specifically for this skylight, and therefore the fuse in the transformer was intended to protect the transformer PRIMARILY from over temp, and only secondarily for over current. But I seem to have blown the fuse by over current. A 55 VA transformer at 24 v translates to a load of about 2.25 amps (ok, 2.29). I think the NTE part would not protect against a transformer current overload.

So, my questions are:

  1. have I analyzed the problem correctly?
  2. Should I pursue the NTE (15A) thermal fuse, or try to find the obsolete Elsen part?
  • \$\begingroup\$ Have you checked that that is the fuse which is open? It seems a little unlikely to me that a momentary short on the secondary would have raised the transformer temperature to 128 °C. \$\endgroup\$ Jan 19, 2018 at 19:07

3 Answers 3


A Current fuse opens above a protected current level defined by some I*t curve.

A Thermal fuse opens above a protect temperature limit which is appropriate for this. It can also protect appliances that are supposed to get hot like coffee pot heating pads.

When the current can be 800% during a startup, it is not harmful to the transformer until the temperature rises above it's protected limit.

This would apply to low energy transformers not kW , MW sized devices where faster protection is needed.

Consider these, available in any appliance spare part dealer. http://www.weisd.com/test/WEISD_TBL_view.php?editid1=NTE8125


That is a thermal fuse, not a current fuse. It opens when the transformer gets above about 128 Celsius, hence the part number. You should replace it with a thermal fuse of equal or lower temperature rating and reattach it to the transformer with solid, permanent thermal contact.

If it were a current fuse it would be a slow blow type. The transformer can handle overloads for a while before it gets too hot. Since overheating is a primary and dangerous failure mode of transformers, protecting them with a thermal fuse is appropriate. Fast blow fuses are used where faults cannot be tolerated for any duration, such as in sensitive microelectronic circuits, and must be cleared as quickly as possible.

  • \$\begingroup\$ A thermal fuse is BOTH current and temperature activated ....a thermal cutout switch is only temperature activated. \$\endgroup\$ Jan 19, 2018 at 19:24
  • 1
    \$\begingroup\$ @JackCreasey - Surely thermal fuses have a maximum operating current, but the intended function is thermal protection. \$\endgroup\$
    – vofa
    Jan 19, 2018 at 19:31
  • \$\begingroup\$ I don't disagree with you, however the thermal fuse has a resistance, and pure current flow can blow it. That may have been the problem for the OP rather than the transformer getting hot. \$\endgroup\$ Jan 19, 2018 at 19:41
  • 1
    \$\begingroup\$ Shorting a transformer secondary is a good way to instantly pop the fuse. \$\endgroup\$
    – user105652
    Jan 19, 2018 at 21:47

110 transformer? you need to get a new 120V transformer if you are on standard US power (120V) otherwise its going to run hot. Usually you don't bypass a thermal fuse on a transformer. There are several safety checks that a repair guy should do before they do that, but usually its better to replace the transformer than to be opened to get sued if the appliance catches on fire.

But to let you know, a thermal fuse is a special type of slow blow fuse.

  • 2
    \$\begingroup\$ i think 110 == 120 in most non-engineer minds... \$\endgroup\$
    – dandavis
    Jan 19, 2018 at 19:37
  • \$\begingroup\$ I don't know how old it is, or where the OP is located, but thank you @dandavis for reminding me that some users here are not schooled in electronics. \$\endgroup\$
    – drtechno
    Jan 19, 2018 at 20:00

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