Replacing ahigh voltage fuse

Hi everybody, I know my question is not related to plc but I need your help.I have to replace a fuse 5Kv & 0.75A, but I can't find one, if i replace it with one 0.75A 600v what effect it will have in case of a short.please help,Thank You very much.

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locus said:

Hi everybody, I know my question is not related to plc but I need your help.I have to replace a fuse 5Kv & 0.75A, but I can't find one, if i replace it with one 0.75A 600v what effect it will have in case of a short.please help,Thank You very much.

Click to expand...

My knowledge of fuses is a bit limited, but what I do know is that fuses have a "normal" current rating (in your case .75A) AND a rating that describes the absolute maximum current that can be safely interrupted by the fuse (ie. the fault current limit). If this value is exceeded under dead short conditions, you have a good chance of doing some serious damage.

It is imperative that at higher voltages you have the correct protection systems in place.

You need to check the impedance of the fuse you intend to replace with. With this impedance value, use good old Ohms Law for the current rating. This will confirm that you cannot use it.

Regards,

Ken R.

If you can list exactly the part number from your old fuse, we can probably identify a good replacement.

If not, we need to know what the application is (for example - primary side of a transformer and that transformer's size).

In any case, fuses come with several characteristics that are important to understand. Some are fast acting, some are inverse-time delayed, some are AC and some are DC, and so on.

With regard to the voltage, typically you can use a higher voltage rated fuse in a lower voltage application with no significant risk provided that you have the right type (fast acting or inverse-time), especially in the event of a true short-circuit.

As far as trying to figure out fuse size using impedence and Ohms Law, I don't know about that. Here in the USA, a fuse is typically sized with respect to its type and then with respect to the load it is intended to protect. The NEC Code offers indepth guidelines for sizing fuses if that is any help.

Steve

It is important to replace fuses with ones having a voltage rating equal or higher than the service they are protecting. This is due to the fuse's ability to open under overcurrent conditions and then stay open.

Fuses with inadequate voltage ratings stand a very good chance of arcing back over and, in the process, destroying the protection they were intended to give.

In physical terms, when a fuse opens, the resulting gap defines how much voltage it can block without arcing. Fuses with an excessively low voltage rating will open but have a smaller gap than is necessary to assure they stay open.

Bottom line: use the proper fuse.

It is my understanding that the current rating of a fuse defines the current that will cause the element to melt from I2R heating. The voltage rating, like any voltage rating, defines the maximum voltage that the insulating materials are designed to handle. At a higher voltage than the rating the fuse insulation may break down, and cause arcing or personell hazard. If you are putting the fuse in a 5kV line, you had better use a 5kV or higher rated fuse. If this is a 480 VAC line, then the 600 V fuse will be OK.

REPLACING A HIGH RATING FUSE.

LOCUS,
I HAVE GOTTEN YOUR MESSAGE.
PROBABLY YOU ARE NOW ANXIOUS.
if you connect in your own opinion then problems occur unless
you read this message:
1.
a.let we assume power supply is 10kv-.75amp rating.
b.so, you must connect fuse to that rating.


c.now you decide to connect 600v-.75amp rating fuse.
that means this fuse can't pass over 600*.75=450watts
in your circuits.so to satisfy 450watt rating total
current will be(maximum)( 450watts=1000*i , i(max)=0.045amp
so load resistance will be r(min)=v/i,r(min)=10000/0.045=222.22k
if you allow less than this load resistance then with equation
p=(v*v)/r, p will exced 450 watts and your fuse will disconnect
from circuit.

conclusion:
So be sure the resistance of load of mininum of 222.22k(ohm).

by - saifullah
RAJSHAHI , BIT,BANGLADESH
3RD YEAR,SIX SEMESTER.
COMPUTER SCIENCE AND ENGINEERING DEPARTMENT.
B.SC.ENGINEERING.
DATE:09/07/03
email:[email protected]

Steve,

Fuses have voltage ratings for the reason mentioned before. If a lower voltage rated fuse is used to interrupt a higher voltage circuit. In this case using a 600v in a 5KV then the nominal current carrying capacity of that fuse will be 6.25 amps not 0.75 amps. Fuse voltage is critical whether fast or slow blow for this reason.

Regards,

Ken R.

Posted bysaifullah99319
conclusion:
So be sure the resistance of load of mininum of 222.22k(ohm).

Click to expand...

DO NOT USE THIS ADVICE

Read DickDV's post above. Do not use the 600V fuse.

Ken: I think you mis-read me. What I said was "typically you can use a higher voltage rated fuse in a lower voltage application" not the reverse. I did, however miss that he said 5KV. I read that as "5KW" for some reason.

Steve

Sorry don't read #post7 ..
Read this carefully.
thanks you.



LOCUS,
I HAVE GOTTEN YOUR MESSAGE.
PROBABLY YOU ARE NOW ANXIOUS.
if you connect in your own opinion then problems occur unless
you read this message:
1.
a.let we assume power supply is 5kv-.75amp rating.
b.so, you must connect fuse to that rating.


c.now you decide to connect 600v-.75amp rating fuse.
that means this fuse can't pass over 600*.75=450watts
in your circuits.
d.so to satisfy 450watt rating total current will be(maximum)
( 450watts=5000*i , i(max)=0.09amp).
so load resistance will be r(min)=v/i,r(min)=5000/0.09=55.55k
if you allow less than this load resistance then with equation
p=(v*v)/r, p will exced 450 watts and your fuse will disconnect
from circuit.

conclusion:
So be sure the resistance of load of mininum of 55.55K(ohm).

/**********************/
if you choose less than 55.55k under above circumstances
then it is a good choose to 5kv-.075amp rating fuse.

Thanks you-
by - saifullah
RAJSHAHI , BIT,BANGLADESH
3RD YEAR,SIX SEMESTER.
COMPUTER SCIENCE AND ENGINEERING DEPARTMENT.
B.SC.ENGINEERING.
DATE:09/07/03
email:[email protected]

Apologies Steve, I mis-read you. Still banging on about fuse impedance, I have been involved in low voltage (600vdc) high current jobs where each line is protected by a couple of fuses in parallel. Hence critical to have fuse impedance matched in those circumstances.

Regards,

Ken R.

EEK! Fuses in parallel on the same line! I have never had to do that before.

I work exclusively at 600V or lower (mostly AC for me, though) but my applications rarely go much higher than 300 amps. Obviously that makes my fuse sizing a lot easier.

My toughest applications (which are really pretty easy, too, if you follow Code) is when I have a whole bunch of motors on one machine fed from a single source and then have to recommend a fuse size to the customer. Even with that I tell them they need to have a local electrician size things in order to assure compliance with their local jurisdiction.

Steve

My Opinion:

5000 volt fuses are nothing to be playing around with. Before you "trust" (in this case) any of the replies here, I would seek help from a fuse manufacturer or the installer of the system, and make sure you are doing the right thing.

Depending on the physical size of the fuse and your application, you may find one at an appliance repair shop. Microwave ovens use 5kv fuses in that current range.

There is some dangerous mis-understanding of how fuses work and what the ratings mean going on here. Just because you have a mathematically correct solution, it doesn't mean that the answer correctly represents the physical reality!

The voltage rating on a fuse is just like the voltage rating on a length of wire - it denotes the dielectric strength of the insulation and the gap between the current entry points after the "fusible" link(the part that melts when a fuse blows) is gone. As Dick and I have pointed out, if you have a higher voltage than specifeid, when the fuse blows you can have arcing between the two ends of the fuse, or even worse before the fuse blows you can have arcing to grounded panel components! This is NOT GOOD!

This is no different than using wire with thin insulation, rated for 600 VAC lets say, on a 4,160 VAC application. It is possible to have the voltage arc right through the insulation to a nearby conductor. That is why you MUST match a fuse voltage rating to the voltage of the circuit.

There are lots of things going on inside the fusible portion of a fuse, most of which I don't pretend to understand. This is particularly true of time delay fuses. However, I know that a fuse melts because of I-squared-R heating. It is the CURRENT that causes the heating, regardless of the voltage applied. That is why you can use a 120 VAC rated fuse on a 24 VDC circuit.

If you have a short circuit or overload, the fuse blows because the total load impedence combined with the voltage available drives more current through the fuse than it's rating. This causes the fusible link to heat and melt, opening the curcuit. The idea is to have the fuse blow before the high current causes heating in the circuit that causes a fire! That is why you MUST match a fuse to the current carrying capacity of the wire in the curcuit.

Changing a fuse rating by the ratio of voltage ratings and resistances is absolutely positively WRONG! Volts times Amps is Watts, and Amps x Amps x Ohms is Watts, and Volts divided by Ohms is Amps, but that doesn't have a thing to do with the selection of the proper fuse.

Locus, if you aren't sure about swapping the fuse with a different one, DON'T DO IT!!!!!!