inverter help

Hi lads after such much needed help and advice my 600w inverter recently packed up so bought a new one on tues gone. Connected it to existing wiring and away it went but on fri gone its blown completely have inline fuse fitted which blew but the 2 fuses in the inverter didnt which is suprising as spose to have protection. Anyway had our sparky check wiring which is spot on then voltage which again was spot on with voltmeter. Was only running a coolbox at time (aldi one) which is 240v and 57w so surely should be fine anyway sparky tried his inverter on my wiring and fired up no prob at all. Have got receipt for this and surely this is a faulty unit as sparky says after in line fuse went the inverters fuses should have went. Anybody help me on this i would be very grateful as dont want to make a ■■■ of myself taking it back. Cheers lads

Take it back for a new one but I’d be checking that the cool box doesn’t have a intermittent fault also