Fuses

[hvac2379]

Can anyone tell me when checking a fuse on a trane rooftop 460V, do you have to take them out to check or can you just shut off the break and check? Also when checking these fuses, i know i will be reading ohms but what am i looking for so i know its good? Thank you for any feed back!

[roaddog]

When I do preventive maintenance on our 480 volt lieberts I do not pull the fuse to check them.

[chiller mekanik]

The basic nature of your question gives me an idea of your experience level therefore I suggest the following procedure.
A)Turn the power off.

B)Verify the power is off.

C)Remove the fuses.

D)Check for continuity, the fuse is either open or closed.

E)Never replace just one fuse, always replace the group.

Are these fuses for the whole unit, compressor section or for the condenser fans or what?
If you have a group of 3 & 2 of them are open & they are for a few condenser motors, you will want to check the motors to find the bad one or it may blow the new fuses, sometimes the motor will open when it fails & takes out fuses then the open motor is the bad one.
Be careful & good luck.

[techrider]

You dont need to shut off the power to check. If you check across the fuse if it is good you should read no voltage and if its bad you will read name plate voltage.

With power OFF you can pull the fuse and you should read no continuity on a blown fuse.

Consider having a seasoned tech show you if uncertain. It is my opinion its a plus to ask for assistance.

[maxster]

if you read top to bottom no problem with them in place(ohms only)if the fuse is good you will get continuity...audible,if it is blown you will see nothing on the meter it will sit at zero.you can also tip them out from the top to read them leaving the bottom in...it is a good habit to pop them to see if there tight in the boot as you reinstall them.buying on of those black plastic/fiber fuse pullers is always handy

[techrider]

If you read above and below my previous post you will be getiing info from techs with alot more experience.

[ericnyc]

live fuses... check voltage from top to bottom...you should read 0, nothing. However you may get some voltage being fed through a transformer that may throw you off especially when your missing a leg.

[flange]

There are instances when you can check them in place, although some times you cannot. if in doubt, kill power and read continuity through the fuse. you never get fooled by checking them out of a circuit.

[mrhvacmechanic]

First, before we check anything! Which fuses are we talking about???? Control or device fuses?????? single or three phase???

If the power is ON and the fuses feed a contactor that is not pulled in, and you read accross fuse with your volt meter, and the fuse is good, you WON'T READ ANYTHING!!!! You may have 460V feeding top of fuse and fuse is good!

Being such there is no voltage comming into the load side of the fuse!ONLY when there is voltage coming the other way will you find an open fuse with a voltmeter.

Be careful!The best and safest way is to kill all power, pull fuse, and check with your ohm meter for continuity thru fuse. As TECH says seek assistance from a quilified tech.

[Edited by mrhvacmechanic on 10-23-2006 at 11:01 PM]

[keepitsimplestupid]

Hey flange. Have you ever seen an intermittant fuse. Drove me nuts only once. Ohmeter out of circut checked OK. Voltage check across fuse revealed fuse bad.

[alpha480v]

Always check for the presence of voltage first, before you use your ohm meter to check for continuity of the fuses. I have seen disconnects that have hot fuses, where someone wired it wrong. They brought the line wires to the load side of the disconnect instead of the line side. So when the disconnect is shut off, the fuses are still hot. I have also seen where the disconnect is so rusted, that it doesn't dis-engage all three phases. One phase might still be hot. Another interesting thing that has happened to me is the breaker feeding the disconnect is defective, not shutting off all three legs. I remember when I was in school learning this stuff, our instructor told us to never trust anyone when they said the power was off. Check yourself.

Always check with your meter first. That's what it's for!

[mikethemech]

Don't take this wrong but if you need to ask then it might be better to take someone who is used to 480V circuits with you..this isn't a tough thing to do but if you make a mistake you could ruin your meter or worse yet get hurt or killed.

Personally I usually check fuses in place, with the power on, I read with my meter set to VOLTS ( NOT OHMS) and check across the output side of the fuses for voltage. You need to do 3 checks to verify 3 good fuses that way.. check fuse 1 to 2, fuse 2 to 3, then fuse 1 to 3. If you read the same ( about 460 volts) then you have 3 good fuses AND you verified you have 3 good incoming phases of power up to that point. If anything seems odd..then turn off the power, check all 3 phases to ground to make sure they are dead ( VERY IMPORTANT..BEFORE you touch anything) and then remove the fuses and test them individually with an ohm meter.

But please.. only do this job if your used to working with this level of voltage, if you aren't then you need someone to show you how to work safely the first few times.

[mikethemech]

A related story..always TEST with a known working voltmeter before touching anything, I didn't once and got nailed real good.
About 10 years ago I was changing a little 3/4 horse 480 Volt motor on an oil circulating pump ( one of a set of two) which was running but noisy. I set the valves to use the other pump , and switched the selector switch to the other pump so the boilers would keep firing. The first motor stopped, the second one started, and I went ahead stupidly with removing the bad motor. I never tested it..heck the motor stopped, obviously it was off.
NOT!!
The problem was that the contactor for that motor had a set of welded contacts and 1 phase was still connected and when I took apart those wires..OUCH! 480 doesn't feel nice, and especially not when your well grounded, lying in a mess of cold sweating oils pipes in a hot steamy boiler room. I've gotten many shocks in my years of work but that was the worst by far..lucky I dropped the wire or I could have died. As it was I sat up, then decided to not stand up for a few minutes, I felt that bad. Nothing permanent..but I sure learned a lesson that day.

[alfe]

Turn power off.

Check to make sure there is no voltage.

Remove fuses and check for continuity. If no continuity, then replace the fuses.

Find cause of fuse blowing. Check the load device.

Rectify problem.

Turn of power and monitor system. RLA should be with nameplate rating.

Job well done.

[alfe]

Turn power off.

Check to make sure there is no voltage.

Remove fuses and check for continuity. If no continuity, then replace the fuses.

Find cause of fuse blowing. Check the load device.

Rectify problem.

Turn on power and monitor system. RLA should be with nameplate rating.

Job well done.