To Fuse or Not to Fuse

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Thread Starter

David Plourde

I have worked with some electrical guys who place and inline fuse on each output from a PLC or other type of controller. I have not formed an opinion on this and was wondering what other people think.
Thanks,
Dave
 
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Kenneth Moore

I have worked with PLC's for years and I can tell you that you ALWAYS want to protect each output with an individual fuse or you will spend lots of money with people like me when we repair your output module do to overload. It is also very easy to replace a fuse but sometimes with different PLC's it can be a task to change cards. I hope this was of some help to you.

Kenneth Moore
The Repair Depot
 
If you truly want to "go by the book" then you either fuse the entire module at the rated current of a single output, or you fuse each output. For example, a module may have a total current rating of 8 amps, but each output is only 2 amps. Going by the book, you probably don't want to limit your total current at 2 amps, so you put an 8 amp fuse on the module supply, and 2 amp fuses on each output. (Some PLC modules have internal resettable fuses so this won't apply.) Sometimes you may just throw the book out and risk the output and only fuse the whole card, and as long as you're not using whimpy wires you'll only damage the PLC hardware in the event of an overcurrent. As I read the code, there is some leeway here in control panels because it is assumed that the panel engineer knows what he/she is doing. Its your call. There may also be times when you do not want to loose a single control point, but rather want a whole group fused together for safety or other sound engineering reasons. It is again your call. But make the call for sound reasons, not just to save a couple of bucks, because at some point in the future you dont want to have your butt hanging because you cut corners for a marginal cost savings if something happens. Use wisdom.
 
I do not fuse each output. I made sure the load is within the rated output,Siemens output cards has short circiut protection.
 
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Anthony Kerstens

>From NFPA 79:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
7.2.1.3 Supplementary overcurrent protective devices shall not be used as a substitute for branch-circuit overcurrent protective devices.
7.2.4.1 General. A control circuit tapped from the load side of the branch-circuit short-circuit and ground-fault protective device(s) and functioning to control the load(s) connected to that branch circuit shall be protected against overcurrent in accordance with 7.2.4.2. Such a tapped control circuit shall not be considered to be a branch circuit and shall be permitted to be protected by either a supplementary or branchcircuit
overcurrent protective device(s).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Interpretation: Branch circuit protection is required. Supplementary protectors are allowed for “tapped-off” control circuits, but are not required. I consider individual points on a PLC I/O module to be "tapped-off" a branch circuit.

I prefer use one fuse per module for isolation purposes. That way, if one module blows a fuse I know which module caused the fault. At a minimum I would use one fuse for input power, one for non-estopped output power, and one for estopped output power.

Generally, fusing "tapped" circuits provides isolation that makes troubleshooting easier. However, fusing each and every I/O point is wasteful.

Anthony Kerstens P.Eng.
 
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Bouchard, James [CPCCA]

I have never done this. Generally the PLC outputs we use already have a fuse and that is enough protections for us. Our installations are reasonably well protected and maintained so there is relatively little damage to the wiring or devices. In other industries that are less careful about their equipment there may be much more damage and an in line fuse would avoid having to change the fuse in the PLC output card.

James Bouchard
 
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Fred Townsend

I used to work in a high volume electronic production facility. There, I got a chance to track the reliability of millions of components. The least reliable component, by a large margin, was a fuse holder. Funny how we forget everything we know about making reliable connectors when we make a fuse holder. The second least reliably part was a fuse. I know what you are thinking. Blown fuses were not counted as a failure. That is what the fuse is supposed to do. The failures I'm referring to were mechanical (vibration), corrosion, didn't blow or blow at wrong current type failures.

To answer you question I do recommend etched copper or poly fuses. The etched copper fuse is a PCB trace restriction between a pair of PCB pads. Etched fuses are great debugging tools. If I want to measure the current I cut the fuse and insert an ammeter or current probe between the pads. Then I restore the circuit by soldering in a jumper wire. Obviously you must be in control of the PCB design to use this approach.

Fred Townsend
 
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James Ingraham

I prefer to fuse each output if possible. The usual reasons for us not doing it are space considerations or the complexity of the wiring. I don't think it's essential, especially since I/O cards are not that expensive. If you are blowing fuses constantly, then there is a problem that should be fixed.

-James Ingraham
Sage Automation, Inc.
 
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Bob Peterson

Pro:
1. Allows a single point to blow its fuse rather than blowing an entire
card's fuse (or some part of it) if a short occurs.
2. Nice way to isolate a single circuit if necessary.

Con:
1. Costs extra.
2. Real fuses take up extra panel space.
3. The small 5x20mm fuses seem to have a lot of DOA problems. I have seen
as many as 1 in ten in a box be open. A real pain in the field.

My suggestion. If you are going to do it do not use the 5X20 mm fuses. Just
MHO from several projects where I had a lot of problems with them.

Bob Peterson
 
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Steve Myres, PE

This is actually required on some automotive industry specifications, and it is common enough that Allen-Bradley actually sells swing arms that have internal per-channel fuses.

I tend to think it's a little excessive, but there are advantages. It sure makes it easy to locate a short in a wire when there is only one wire downstream of the blown fuse.

In general, the factors I would consider would be:

Do the outputs terminate in the same control cabinet with the PLC, as opposed to running half a mile across the plant in pipe? If so, they are pretty immune to physical damage, and may not need individual fusing.

Is the nature of the process such that something useful can still get done if that output is disabled because its fuse is blown? If the loss of the function of the output will shut the machine down anyway, where is the benefit of keeping the rest of the outputs on the module working?
 
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Jeffrey Eggenberger

It would depend on what I am driving. If just a relay, then no. if a
solenoid, then a 1 amp resettable breaker, or fuse works wonders.

Jeff.
 
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Hello David
Fusing each PLC output is usually a good thing to do but you have to look at your particular application and the specs of your PLC output module.

The reason for installing a fuse would be to protect the output module from damage in case the load goes short circuit. This is fine for relay outputs if you size the fuse correctly. The module data sheet usually has a recommended fuse size.

It is difficult to protect solid state outputs with fuses although you could try semiconductor fuses. Some solid state PLC output modules have built in overload protection.

You should look at your loads and determine if they are likely to short circuit. Standard incandescent indicator lamps are a good example. They will eventually burn out and in doing so may present a momentary short circuit to the PLC output. You could overcome this problem by
1. use relay outputs and fuses or
2. use overload protected outputs or
3. use LED indicators instead of incandescent lamps

I usually try to avoid fusing all outputs because it adds cost and complexity and can make fault finding difficult. I prefer to have one fuse or circuit breaker for section of the control circuit (perhaps 4 outputs). I use relay outputs for AC loads and overload protected solid state outputs for DC loads.

Another method is to use interposing relays for all outputs. These can be a very good solution. You can use 24V DC high density output modules with one fuse per module with each output connected to an interposing relay with LED indication. Allen Bradley and Phoenix Contact make "terminal" relays for this purpose. Using this method can reduce the size and cost of your PLC as well as protecting the output module from short circuits in the final load.

Regards
Jamie Downs
[email protected]
 
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Our company policy is to fuse all external I/O (any I/O connected via terminal strip to field devices). A fusible terminal with blown fuse indication costs approx. 3 times more than a regular terminal. The increased cost of a fusible terminal is always worth the money. From a maintenance perspective, fused terminals are priceless. On time critical process lines, a grounded proximity switch can be quickly and easily diagnosed and repaired without shutting down equipment. We also specify fuse sizes. Our plant has several thousand I/O, and only four fuse sizes. Electricians always carry a packet of fuses with them, ready to handle an emergency. I noticed that you only indicated outputs. Don’t forget the inputs.
 
At our facility we fuse each output and each "field hot" that returns to an input from contacts in the field. We use the din rail style fuse holders with blown fuse indicators. Good troubleshooting tool and limits a fault to one discreet point when you have a short. (Keep eggs in separate baskets.)
 
Dave
god created transistors to protect fuses.
if u r going to fuse make sure it is a solid state fuse or if common point is 8A and ea out is say 2 amps fuse <= 2Amps that way the fuse may beat the pcb track in the PLC

richard
 
Years ago PLC manufactures USED to fuse each output within the module and they were user replaceable. Today, with reductions in size and cost, there is only 1 fuse per "bank" of 4 or 8 points. The fuse(s) are generally soldered to the PC board and many do not look like a fuse.

I would have to say that your "Electrical Guys" are right on!
 
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Bouchard, James

One problem that occurs when you add outboard fuses on outputs is that you have no feedback to the PLC. This lack of feedback can cause problems in the logic or require extra programming to detect cases where the sequence has stalled because a fuse opened the circuit. Even more problems on inputs. To the point where we brought the various voltages feeding I/O to directly from the fuse or circuit breaker to an input that we expected to be always on. If it went off we knew there was a blown fuse and did a graceful shutdown.

James Bouchard
 
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I have noticed that many of the responses from the engineering group think it wasteful to fuse individual I/O. Some of these responses are from individuals that obviously have no field maintenance experience. Assuming all field wiring is terminated at a terminal strip, (if field devices are wired directly to an I/O card, then the panel was not designed well, IMHO) then why not use terminal fuses?

If relying on an I/O card’s internal fuse protection, even if the card design has individual I/O point fuses, you still need to shut down the equipment (DOWN TIME) to replace the fuse.

As far as the reliability of the fuses and fuse holders, you get what you pay for. Most manufactures offer high quality fused terminals that are the same size as their regular terminals. I have been “burned” many times by A-B 1492-CE6 fuse holders. They do not hold up well to vibration and tend to corrode when subjected to high humidity.
 
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Hurd, David L \(GECP, MABG, 053698\)

I prefer to use an inline fuse on outputs that drive valves or relays because it's easier to replace. The fuses on most PLC output cards require shutting down the PLC and removing the card to replace.
Dave
 
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Michael Griffin

On February 14, 2003, David Plourde wrote: <clip>
> I have worked with some electrical guys who place and inline fuse on each
> output from a PLC or other type of controller. I have not formed an opinion
> on this and was wondering what other people think.
<clip>

Everyone is giving different answers, because this is a subject which is application dependent. In many cases fuses are not useful. I believe that this installation practice originated in the days when 120VAC relay outputs were common. It is less useful with 24VDC transistor outputs.

If an end user is concerned about short circuits in their output loads, then they should use a model or brand of PLC which offers short circuit protected output cards. Fuses are a poor substitute for this feature.

I believe I saw at least one reply recommending fuses in each input. I can't imagine what use this would be for sinking inputs and sourcing inputs are designed to be shorted out as part of normal operation.

Fuses should be present for protecting wiring and other similar devices (e.g. terminal blocks, etc.). Ordinary 1-1/4" fuses (and similar devices) are of limited use in protecting semiconductors. They are perhaps more useful in protecting relay cards. However, relay cards tend to have a limited mechanical life and fail without any fault occuring in the load, so I can't imagine anyone who wants fuses everywhere happily accepting relay cards.

Supplimentary protectors (miniature breakers) are completely useless for this purpose. In Canada, you are not even allowed to use them to protect wiring, although I know this is common practice in Europe and the USA. All of these devices have such wide manufacturing tolerances that it is impossible to predict whether they will trip before your wiring burns up. I find it difficult to believe they could have any use in protecting transistors.

With smaller machinery, the total output load seldom exceeds a couple of amps (air pilot pneumatic valves draw very little current). In the even of a short circuit in a load (or sensor) in this type of machinery, it is very unlikely that a properly sized fuse will blow before the power supply goes into fold-back. In fact, while I have seen shorted proximity sensors pulling the whole machine down, I cannot recall ever seeing this blow a fuse. The power supply fold-back circuit is simply many times faster than the fuse curve. Blown fuses are more commonly caused by defective fuses than failed loads. The situation may be different with large power supplies.

The point of the above is that fuses and circuit breakers are not magic. Putting a fuse in series with a transistor will not necessarily do anything useful. However, if a customer asks for them, then by all means provide them. Some people feel you can never have too many fuses.

Often though it is much better to use the money and design effort you would spend on the fuses to improve the servicability and reliability of the machine in other ways. Many electricians like to have extra fuses not for the sake of the fuses themselves, but rather for the convenience of using them as "switches" to allow them to isolate groups of circuits for troubleshooting purposes. There are other ways of providing this and other useful features.

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Michael Griffin
London, Ont. Canada
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