Inrush Current

Sounds like real fun. Had the same problem (management understanding technical stuff) when our reflow oven (also 350 Amps per phase at 415 Volts) kept blowing breakers on cold start.

May I suggest that you need some way to "sequence" the loads during the cold start. Maybe an oven program that starts with a setpoint at room temperature (so the heaters do not come on) and that should enable all your motors to start turning and overcome the machine inertia.

Once that part of the hardware is initialised, then ramp the heaters on group by group.

I guess from the information that you've provided you would have access to a clamp on meter to measure current in each load portion during cold switch on.

I'm guessing that if you put a breaker switch in front of the two 150 Amp heater transformers, you will be able to isolate those and switch them on independantly.

By the way, what sort of oven is it? Ours is a reflow oven for electronics manufacture. Fortunately we only face cold starts two or three times per year.

I appreciate your response and thank you ... I have tried everything and yes I Have had at one time every fuse out and trying to just start up one unit up and when I throwed in the temporary disconnect, the wires feeding the primary side of the transformer rattled or jerked and blowed the fuse....All I know about the oven is it is a CM manufactored in New Jersey in the USA which is a 92 model....I even called the service engineer there and he said we had to have a short to ground in the transformers but I have checked it and nothing.....I have ran the units from another 480 power source which comes off the same switgear and it worked fine ....At that time the plant was running and the system was loaded . Saturday the plant was down and nothing running and my God everything we did it blowed the fuse...so it makes me fill like I am in grammer school again because I have been in electrical work now 35 years and thought I was a pretty good trouble shooter ....but I have my doubts now......You Know just some of the answers you get they know how to cut deep...I appreciate you time and thank you.

this is for a wire manufactor......for guide wires and towers and whatever else they do.

You could use a "soft start" on the motors or if variable speed is needed a VFD will also limit the inrush and you can program it to whatever percent of full load amps you desire.

If I remember correctly each section would have a drop off of demand when they were within 1/4 of operating temp. and the switching system was set to engage the other parts of the ovens as they reached the 3/4 point of the set temp.

The last big ovens I work around maintance installeda system of switches so that on start up one section got power at a time and they installed a green light which would signal when the oven was at operating temp.

That sure bet the old ones we had to hand start each section by hand with mostly our experience to tell us when to engage each section so we did not have the same problem your having.

We used them for curing teflon and to cure taped nickle plated copper cables for aircraft power supplys.

I am no expert at this, but it seems to me that if you could activate the two transformers separately instead of simultaneously, you might relieve the problem. A transformer is going to look like a dead short to ground for the first 1/60th of a second, just like a motor...