Just to add to James' post. What voltage and for how long are you seeing this that causes the device to lock up?
This is not "normal" but if the input is out of spec then you will have unknown behaviour and should correct the power but I assume this is caused by a generator or something so if you cannot correct the power then you should install a low voltage cut off.
Most battery isolators have a timer to allow for fluctuations in power. In your case you would want an isolator that cuts out immediately if you can find one.
Alternatively ask your electrician to install an under voltage monitoring relay. This will cut out immediately (normally within 0.1-0.5 seconds) when the 24VDC drops below a certain point. You can get them from Phoenix Contact, Eaton etc.
Here is an example of one: http://uk.rs-online.com/web/p/monitoring-relays/7300509/
This has a Drop-out voltage of >30% of minimum rated supply voltage, which is 24VDC so it would cut off at 16.8VDC.
Obviously the WR21 is rated from 9-36VDC so the device would be protected from under voltage and restart once the power returns to a normal voltage.