Update on ISO20934
Solutions for Breaking Capacity Tests for Automotive Fuses
In a past "Solution Notes" article I covered solutions for breaking capacity tests for automotive fuses. In this article, I will share information on recent developments in the process to reform automotive fuse standards.
PRF (Proof) of ISO20934 Standard Now Released
The standard in question is the ISO/DIS 20934:
Road vehicles─Fuse-links with axial terminals for use in 48V networks─Types SF36-48V, SF51-48V and SF56-48V, which as of May 2018 was still a Draft International Standard.
The scope of application of this standard, which is based on ISO8820, is described as follows:
The ISO20934 applies to road vehicles with a nominal voltage of DC48V, a rated voltage of DC70V, a rated current of 30A-500A, and fuse links intended for use with electrical systems with a breaking capacity of 2500A.
The updated standard (as of February 2019) is set forth below. While not yet finalized, ISO20934 has now been upgraded from a Draft International Standard to a Proof of a new International Standard.
Road vehicles - Fuse links with axial terminals for use in 48V networks -
Types SF36-70V, SF51-70V and SF56-48V
Before becoming a standard, an ISO proposal goes through each of the stages below:
(Preliminary Work Item), New Proposal, Approved new Work Item, Working Draft, (Committee Draft), Draft International Standard, Final Draft International Standard or Proof of a new International Standard, International Standard (Publicly Available Specification, Technical Report, Technical Specification, or Report)
Therefore, the next stage after Proof (PRF) is International Standard (ISO).
Electronic Loads Not Permitted in Constant-Current Mode
While the standard makes various stipulations in respect to fuses, I was most interested in breaking capacity. The description of the testing parameters includes the phrase, "The usage of an electronic load in constant-current mode is not permitted."
This was a problem. In my "Solution Notes" article on solutions for testing the breaking capacity of automotive fuses, I discussed extensively the issue of accurately setting the slew rate of the electronic load in constant-current mode.
Using Constant-Resistance Mode
After considering the problem for a while, I had a brainwave. Why not use the electronic load’s constant resistance mode?
The test circuit specified in the standard for testing fuse breaking capacity comprises a power supply, a resistor that determines the current flowing through the circuit, and an inductance that determines the rise time (time constant) of the current.
I decided to configure the electronic load not in constant-current mode but in constant-resistance mode. I also decided to simply use the electronic load’s sequencing function to achieve a rise time corresponding to the constant time specified in the standard. I then conducted an experiment to test my circuit.
The waveform of the test current applied to the fuse in the breakage current test is determined in accordance with the figure below. (Figure 1)
As building a 2,500A power supply would have been unrealistic, to put it mildly, in my experiment I set IB at 70A. (Figure 2) I used the sequence function of the electronic load device in constant resistance mode.
I successfully recreated a test current having the prescribed rise time of 2ms.
Once the new standard comes into force, all fuses for overseas applications will need to be tested to the new standard. This will impact Japan’s domestic automotive fuse standard, JASO D612, which is based on ISO8820.
I encourage you to enquire with Kikusui for any fuse-testing requirements. I believe we will be able to provide a convenient solution that allows fuses to be tested with the electronic load device set to either constant-current mode or constant-resistance mode, as necessary.
*By way of background information, the circuit diagram for the fuse breakage test circuit is reproduced below. While this has been taken from the JASO D612 automotive standard, a similar circuit is used for the ISO standard.
Fuse Breakage Test
The fuse breakage test is performed using the circuit set forth in Fig 2, whereby power supply Q is set to UR (+2/0)V, a jumper or dummy fuse SL is inserted as indicated, switch SW is closed, and inductance L and resistance R are adjusted such that breakage current IB is within (+5/0)% of tolerance and the time coefficient is 2±0.5 ms. Finally, switch SW is opened and jumper or dummy fuse SL is removed. Next, fuse F is inserted and switch SW closed as we wait for the fuse to break. For a 30 second period after the interruption of current, rated voltage UR is maintained while leakage current is measured.
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