Dual +/- Voltage Supply fusing. Both Legs?

skyfox

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Does anyone know when a sensor is using a Dual voltage Supply (+/- 15VDC) for it's source power, do both +15V and -15V supply to the sensor require fusing or, can a single Fuse be placed in series with the common line?

What about the signal line itself? OK to just fuse the signal line and not the source Line(s)?

What is the recommended practice. I typically come across some installations with out fusing on either of the lines going to a sensor. Most, have worked for years, but that is not to say I condone the practice.

Thx.
 
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Think about fusing for a second. Imagine all three had fuses on them. +15V, -15V and 0V. Assuming that 0V is connected to ground.

What happens when +15V shorts to ground? What happens when -15V is shorted to ground. What happens when 0V is shorted to ground?

Now imagine what happens if we remove the fuses from +15V and -15V. The power supply and wire are no longer protected by a fuse. Sure fusing 0V will protect the power supply in the event of an overload but not in the case of a short circuit to ground.
 
Typically it is best practice to fuse as close to the Power Source as possible.
If you have a hot going out to a sensor and coming back as a signal then you fuse the Hot or voltage source side only.
The +15 is a power source and the -15 is a Power Source so one would fuse both.

To be more specific you want a fuse where the Current originates.
 
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Two fuses. +15 and -15.
Do not fuse the 0V line.
Do not fuse the signal line.
If it's only a few sensors, you probably don't need fuses at all. Switchers (SMPS) have automatic shutdown on overloads.
 
Thank you all.

Wow. Three different perspectives. That is unless I misunderstood Timbert's answer. For some reason or another (I tinkered around with Heathkits few years back. Few, Few years back) I seem to be drawn to keithkyll answer/reply.

Suppose for a moment..... I could not care less about the $250 Power supply. My sensor is a $6000.00 a piece device. (an MKS Pressure Transmitter thank you)

Now what do I do to protect it in this scenario? In other words, who do I FUSE?
 
All three answers are the same. Fuses protect the power supply, not the sensor.
Check to see if your power supply has overvoltage protection. Probably not on +/- 15.

You might need to use the power supply from MKS to maintain warranty.

I haven't been inside an MKS head for several years. Think they have overvoltage protection. Post model number, and I'll see if I can download a schematic.
 
Power supplies usually have electronic over current protection or a fuse as part of the power supply. This is what protects the power supply.

If you add fuses between the power supply and the loads they have two functions.

First, you are protecting the wire. If the power supply can provide more current than the wire is rated for, you use a fuse to limit the current to the wire's rating. This is to prevent the wire from burning and starting a fire.

Second, if you have many devices powered by the same supply and you would like a single failure to not take out power for all the other devices, individual fuses per device will solve that problem.

While these fuses do limit current to the devices, if the device fails in some way that causes it to draw excess current the fuse can't protect the device, the device has already failed.

To the original question: If grounding the wire will cause current to flow, fuse it.
 
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Suppose for a moment..... I could not care less about the $250 Power supply. My sensor is a $6000.00 a piece device. (an MKS Pressure Transmitter thank you)
As already said, fuses protect wires and prevent short circuits from becoming hazardous.

In this case, you may want to consider a two-pole circuit breaker. Not because it will protect the transmitter in the event of a short circuit but rather because a fault will remove all power from the transmitter rather than leaving the sensor with only one voltage.

I'm sure that manufacturer will have a recommendation on whether is precaution is necessary.
 
Thank you all.
Suppose for a moment..... I could not care less about the $250 Power supply. My sensor is a $6000.00 a piece device. (an MKS Pressure Transmitter thank you)

Now what do I do to protect it in this scenario? In other words, who do I FUSE?

Your transmitter should be fine without any fusing. The only time a fuse would do anything to protect your transmitter is after it has already malfunctioned. It is designed to take a voltage from a power supply. The power supply could supply 0.1 amps, or 1000 amps. The transmitter adjusts the devices internal resistance to control the current flow and the current available doesn't matter (as long as it is more than the minimum needed by the device). Many times individual transmitters are fused with a small fuse so that if the transmitter fails, it blows its small fuse and doesn't take out the big fuses on the power supply so other transmitters still get powered and still function.
 

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