Carrier 30GX chillers

[Burrah Boy]

Hi guys, I work at a site that has five of these units with 3 compressors per unit. Over the years I have done various repairs on these units where I have had to remove the refrigerant, carry out the repair, pressure test and vac (obviously). However, on nearly all ocassions that I have carried out this procedure I have had fuses blowing on me. Either one of the 250amp supply or all three of the 120amp fuses on the main bus bar(there are six fuses per compressor but only the three on the hard wired, not the soft start side). This has beaten me around the bush with various electrical checks and test, but to no avail. I happened to speaking to a bloke from carrier and mentioned this to him. He asked me if I electrical isolated the main supply BEFORE i pulled a vacuum and I said no. He said that was the fault but couldnt explain why. He said they at Carrier were issued a bulletin saying that pulling a vac without totally isolating the incomer would blow fuses but no technical explanation why. So once the system is in a vac state, BANG! I cant understand the reason behind this. I have never had issues like this on other chillers. The reason I dont isolate the system is to keep the oil seperator heaters on. I have spoken to other work mates who have also had this problem happen to them.
Any Sages out there willing to give me technical reason and explanation?
Thanks!

[james10]

You dont have to isolate the power completly however you do need to pull the compressor fuses before evacuating out the system this way you can keep the oil heaters on
i'm not sure why this happpens perhaps the same reason you cant megg a compressor under vacuum
http://www.refrigeration-engineer.co...ad.php?t=25294

[troymedhurst]

Hi I have heard thatit is due to oil displacment, and I thought about this and all i can think is, that if the dielectric value of the oil is low it could possibly conduct and short terminals.
I have read somewhere that copper oxidisation i.e brazing without nitrogen can reduce the oil dielectric. I am still not totally sure? hopefully someone who really knows will post a reply and lay this one to bed

[lowie06]

Hi there, if you don't drop power to the compressor, the oil heaters with cause whats known as "stastic tracking" this occours when the comp is in a high vacuum state, with power on the system it can track between the windings of the compressor, causing a dead short, this can damange the comp but most likely will just pop the breaker or fuses.