Allegro current sensors on 220 VAC mains

Question

A friend is designing a power meter for 220 VAC mains and is considering Allegro current sensors like the ACS37002 or ACS71240. Those sensors put a wire with a sub-ohm resistance in line with the current and measure the magnetic field with differential Hall-effect sensors. Low resistance means low wattage even with high current, so those components have tiny footprints, like 8-pin SOIC. They also feature isolation of several kV between the measuring and the measured side.

Something feels wrong to me about putting a bare 8-pin SOIC in line with 220 VAC. Every other component on this side of the board is as bulky as you would expect, and then there is this chip. I would feel more confident if I saw a power TVS diode in parallel, but there isn't any. I can't find any relevant Allegro application potes. After reading relevant datasheets extensively, my main concerns are:

1. If the load becomes a short circuit, the high current will likely make the component explode. Then, any isolation claim will be void and the measuring side may be connected to mains potentially destroying it and anything connected to it.
2. There's a fuse in line with the load, but then it becomes a race of who will melt first which for me isn't straightforward, especially since between the sensor and the fuse there's a choke. You can replace the fuse, but can you be confident that the sensor is intact? I also don't feel confident that repeated short-time exposure to higher currents than specified won't eventually change the isolation properties of the chip. Motors, for example, can create currents of various sizes and shapes that will not blow a fuse.
3. Some of those sensors claim MHz-level bandwidth. The PCB designs I often see, including Allegro's Evaluation Bare Board (a 6-layer x 2 oz copper beast) don't seem to try to minimize trace parasitic capacitances. The distance of 8-pin SOIC and the size of the pads necessary to handle large currents make it hard to design a PCB that handles high frequencies well. I guess there are a few concerns about high frequencies: a) the measurement on higher frequencies might be less accurate, b) hall sensor's functionality requires the absence of a Faraday shield, and it might be hard to prevent high frequencies from coupling to the power lines of the measuring side unless you add ferrite beads. Dimers, motor controllers and switching mode power supplies can create spikes with high frequency components.

How can the Allegro current sensors be properly used on circuits connected to mains power, while making minimum assumptions about the load?

Indicative Distances on Allegro's Evaluation Bare Board and pin distances from the ACS37030 and ACS37032 Datasheet.

Answer 1

I used one of these in a prototype 24 volt system with a 40 amp hour Lithium Ferro Phosphate battery as the energy source.
A short circuit incinerated the current sensor, destroyed the isolation and applied 24V to the low voltage electronics. Fortunately, damage was minimal.

While it is probably possible to design a system so that the risk of catastrophic isolation destruction is minima, if I was using these in future I would provide an isolated low voltage system at the sensor output that was capable of surviving high voltage application. This is relatively easy as you are dealing with very low current at this point.

Answer 2

Some of the ACS37002 parts can handle 133A, and has a 1mΩ resistance through the IP+ channel. So if you were to dissipate the power from 66A in the part, it would be 66mW. The worst case thermal resistance is 8C/W so this would only be ~1C temperature rise, the part won't even flinch at that much current put through it.

The device can only measure 133A, so you must limit the current with whatever margin you feel appropriate and never exceed that current above that value although the part could probably take more abuse since it's just a conductor. So set the fuse to something like 133A with whatever margin you need.

You do need to provide proper creepage and clearance, you design should use a standard like IPC-2221B, whatever NRTL you are passing your design through should be able to comment on what your specific standards are for saftey.

Even if you aren't going to sell this design, you'll need proper creepage to avoid arcing, which means if you have two traces on the PCB at 220V (like a high voltage 220V an low voltage), you'll need 1.25mm of spacing.

Source: https://resources.altium.com/p/using-an-ipc-2221-calculator-for-high-voltage-design