How to protect current sense IC from current transients

Question

I'm planning to use a ACS712 current sense IC to sense motor-current in a project:

http://www.allegromicro.com/~/Media/Files/Datasheets/ACS712-Datasheet.ashx

The device I'm building uses a relay to control 230V to an electrical motor. I want to be able to deliver 2A to the motor.

I want to sense the current to be able to give a ballpark power value, and also to be able to detect short circuit or open circuit.

I've heard that short-circuiting the 230V mains, even in a residential area, can possibly cause currents up to or above 1000 A. This of course means that a fuse in the building will blow.

However, even at 1000A, it may take some time for the fuse to blow. I'm worried (or convinced) that the ACS712 will be obliterated by a 1000A surge, even faster than the fuse can blow (a couple of milliseconds?).

What could I do to alleviate this? I could put a 10 ohm resistor in series with my motor, but that would still put approximately 20A through the resistor, and I'm worried that the resistor will burn out before a 10A fuse will. Also, the resistor would definitely burn during normal operation.

What would be the right way to solve this?

Answer 1

Part of the answer is in the datasheet.

The unit can handle a 100ms single pulse of 100A.

This equates to some serious power path between the pins that are measured inside the device. This power path will very easily "win" from a 10A fast fuse. A fast fuse at 10A is still a tiny little wire, that could never stand 100A for any more than a fraction of a ms.

So if you fuse it in your device with a 10A fuse as you imply, you've done your job. The only way you can then destroy the device is by replacing the fuse by a piece of tin foil and anyone doing that is asking for problems anyway.

Other than that care must be taken to keep the 230V at least 2mm (advised 3mm) away from the low voltage parts.

These little current sensors are actually designed to be incredibly rugged, just so you can put them in any power path and not worry about voltage drops and power dissipations.

If you're still worried, you can add a 1ohm resistor at a few watts, but it'll be more susceptible to damage than your sensor.

Answer 2

If i may, from my perspective current sense components are usually the most robust part of the mesh. They themselves may be very well used as quick overcurrent detection. If you have control on the current (if it's a motor drive, for instance) such detection will allow turning off any switches you have, and save wires, connectors, PCB, etc.

Specifically, there are now hall effect sensors with "fault" output, so you don't even need comparators.