SCXI Fuse Information

Issue Details

Where can I find information about the fuses for my National Instruments SCXI chassis?  I suspect that one is blown and I'm not sure what type to use or how to replace them.

Solution

Every National Instruments SCXI chassis has three user accessible fuses on it that protect the chassis from high voltages. If you suspect that the chassis is not functioning properly you should first determine if the fuses are good. There is one main power fuse and two auxiliary backplane fuses that protect the analog input modules. This KnowledgeBase provides information about diagnosing fuse problems and the correct fuse for the chassis. 

Fuse Diagnosis
In general, the SCXI chassis fuses should never blow and are provided simply to protect the chassis and its modules. Diagnosing fuse problems can save you a lot of time and money. Below is information on the three SCXI chassis fuses and how you can determine if they are blown.

Main Power Fuse 
The SCXI main power fuse is located next to the voltage selection wheel on the back of the chassis. You can visually inspect the fuse to determine if it is blown. You can also use an ohmmeter to check for continuity through the fuse. To replace the main power fuse, follow these instructions:

1. Power off the chassis.
2. Remove the power cord from the power entry module.
3. Using a flat blade screwdriver, pry the door to the voltage selection wheel open from the top.
4. Pull out the fuse drawer.
5. Remove the fuse.
6. Install the new fuse in the drawer.
7. Reinsert the fuse drawer in the right side slot with the arrow pointing to the right.
8. Close the door.
9. Reinstall the power cord.

If the SCXI main power fuse continually blows, you need to make sure that the correct fuse is installed. If the correct fuse is installed for the chassis, remove all of the modules from the chassis. If the chassis has the correct fuse and there are no modules installed in the chassis, but the main power fuse still blows, return the chassis to National Instruments for repair. You can find replacement fuses for these main power fuses at most hardware stores. You can also purchase these fuses from National Instruments. Refer to the table below for information about replacement fuses for the main power fuse.

Auxiliary Backplane Fuse 
The SCXI chassis also has two fuses that protect the analog supply lines to the modules. These two fuses are located behind the fan that is closest to the power supply. The fuses are usually green in color and resemble a resistor or capacitor. To access these fuses, follow these instructions:

1. Power off the chassis.
2. Remove the power cord from the power entry module.
3. Remove the appropriate fan from the rear of the chassis. Be careful when removing the fan so that you do not break the power wire for the fan!
4. The fuses are located on the SCXI printed circuit board backplane. The fuse marked with a copper + on the backplane is for the positive analog supply, and the fuse marked with a copper - is for the negative analog supply. To check whether a fuse is blown, connect an ohmmeter across the leads. If the reading is not approximately 0 ohms, replace the fuse. Please refer to the table below.
5. Using a pair of needle-nose pliers, carefully extract the fuse.
6. Take a new fuse and bend its leads so that the fuse is 0.5 inches long, the dimension between the fuse sockets, and clip the leads to a length of 0.25 inches.
7. Using the needle-nose pliers, insert the fuse into the socket holes. 
8. Repeat for the other fuse if necessary.
9. Reinstall the fan to the chassis.

You might mistake a chassis failure as a problem with the chassis, when the problems might actually be due to a bad fuse for the analog supply lines. If a chassis is auto-detected, but the modules do not appear to be responding correctly this is often a problem with the auxiliary backplane fuses. 

NOTE: You can purchase replacement fuses from Littelfuse. You can visit Littelfuse's homepage using the link below. See this document for more information on the 0251 fuse series, including naming conventions information listed on page 4.

SCXI Fuse Information
The following table lists the fuse information for each National Instruments SCXI chassis:

SCXI Chassis Type	Main Power Fuse Quantity	Main Power Fuse Type	Backplane Fuse Quantity	Backplane Fuse Type
SCXI-1000 120 V	1	1/2 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 220 V Swiss	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 240 V Australia	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 240 V Europe	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 240 V N. American	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 240 V U.K.	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000 100 V Japan	1	3/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1001 120 V	1	3 A 250 V 5 mm X 20 mm MED-BLO	2	4 A 125 V Littelfuse  0251 004MRT1L
SCXI-1001 220 V Swiss	1	1.6 A 250 V 5 mm X 20 mm SLO-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1001 240 V Australia	1	1.6 A 250 V 5 mm X 20 mm SLO-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1001 240 V Europe	1	1.6 A 250 V 5 mm X 20 mm SLO-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1001 240 V N. American	1	1.6 A 250 V 5 mm X 20 mm SLO-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1001 240 V U.K.	1	1.6 A 250 V 5 mm X 20 mm SLO-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1001 100 V Japan	1	3 A 250 V 5 mm X 20 mm MED-BLO	2	4 A 125 V Littelfuse 0251 004MRT1L
SCXI-1000DC Chassis	1	6.3 A 250 V 5 mm X 20 mm TIME-LAG LOW BREAKING	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-1000DC (Continued)	1	3.15 A 250 V 5 mm X 20 mm TIME-LAG LOW BREAKING
SCXI-2000 120 V	1	1/2 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 220 V Swiss	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 240 V Australia	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 240 V Europe	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 240 V N. American	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 240 V U.K.	1	1/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L
SCXI-2000 100 V Japan	1	3/4 A 250 V 5 mm X 20 mm SLO-BLO	2	1.5 A 125 V Littelfuse 0251 01.5NRT1L