FAQs from Rohde & Schwarz

HMC8015 maximal inrush current


I want to perform regular power measurements using my HMC8015.
In terms of the current measurement path: When starting my DUT the inrush current is much higher than these specified 60A peak value (CF3: 20A / Peak +/-60A).
Can you tell me a formula to calculate the so called pre arcing time (I²t) to avoid the internal fuse to be blown?


The current measurement path for the higher measurement ranges (500ma --> 20A) of the HMC8015 contains the following main components:

- 10 mOhm Resistor (Maximum peak current: 150A )
- Fuse FR10GR69V20 (690V AC / 20A)

The most important component is the fuse in that case. Following the specifictions in the datasheet of this fuse, the 690 V / 20 A type has a Pre-arcing definition of 69 A²s.
Let's assume the peak value for the inrush current would be 100 A and calculate the maximum time our fuse can be exposed to this current.

Pa = Pre-arcing time in seconds in A/s
Pa = I² * t --> t= Pa / I²
t = 69 A²s / 100 A² = 69 A²s / 10.000 A² = 6,9 E-3 s

So the maximum time we can expose the fuse to 100A is 6,9 ms.

Some other important informations in this context:

- It is also possible to measure currents using the curent clamps HZC50 (30 A / 500 A peak) and HZC51 (1.000 A / 2.000 A peak).
- If the current fuse is broken, the device must be sent in for repair and calibration. This fuse will have influence on the quantity to be measured.