All,

A bit of an explanation before my question.

I recently had a customer request circuit breakers in place of fuses in a control panel. I made the usual explanations for the need for short-circuit protection.

The distinction that needs to be drawn is the difference between UL 489 circuit breakers and UL 1077 supplementary protectors. Circuit breakers are suitable for branch-circuit short-circuit protection whereas supplementary protectors are not.

The usual Allen Bradley supplementary protectors are not suitable for short-circuit protection and I have always used them in conjunction with a fuse. Misapplied they are quite dangerous. To exemplify this, the Electrical Safety Authority (Ontario) published Safety Notice #99-03 regarding supplementary protectors after a
maintenance worker received second degree burns while resetting one such device in a control panel. The fault still existed when the device was reset and it failed catastrophically.

I always used fuses within control panels for short-circuit protection because suitable circuit breakers were not available in the DIN rail mount style or in sizes less than 10A. What is
available is the usual molded case or panel board plug-in type.

However, I have found that recently Merlin Gerin (Schneider) has released DIN rail mounted UL 489 circuit breakers in sizes 0.5A and up.

Now the questions.

Knowing this, can anyone suggest why I should continue to use fuses?

Does anyone know of OEM's other then MG to offer UL 489 breakers in a DIN rail mount, less than 10A?

Anthony Kerstens P.Eng.

 

Hi Anthony.

First:
I am not quite sure about north american regulations, but IEC regulations require that all Short Circuit protection devices must be able to properly disconnect the possible short circuit current where they are installed.
All short curcuit protection devices have a short cuircuit rating - typically in kA - kiloamps (not the same as the normal current rating).
Everywhere in an electrical installation the short circuit current can be calculated. This can be a bit complicated, but can be simplified by getting the short circuit level at the point of entry in an installation (the electrical power provider must inform this). Down stream from this all short circuit devices must be able to disconnect this short circuit current.
Some fuses and circuit breakers have current limiting capabilities. That means that they can decrease the short circuit level down stream from the fuse or circuit breaker.
The accident that you descripe indicates a serious error in the design of the panel. The rated short circuit level of the circuit breaker was not sufficient for the actual short circuit level.

Second:
Circuit breakers get better and better. They have good short circuit breaking capabilities and current limiting capabilites.
But because of physics you can say that circuit breakers are good for small to medium currents. For large to very large currents fuses are still the best.
Just look in some catalogues for circuit breakers and see how the current limiting capabilities get weaker and weaker the higher the rated current.

Cheers,
Jesper.

 

I am not familiar with the particular circuit breakers you mentioned, but miniature breakers have traditionally had a bigger problem with overload protection than with short circuit. The manufacturing tolerances on small breakers may be quite large which affects how long a circuit may be in overload before the breaker trips. When supplimentary protectors were allowed as overload protection, it was not unusual to see the load (and occassionally the wiring) burn up even if the breaker was "correctly" sized. You would need
to look at the trip curves from the manufacturer if you are concerned about overload.

As well as short circuit and overload detection, there has also of course been the problem with short circuit interrupt capability. This may or may not be a problem depending upon short circuit current availability. If you are concerned with DC loads, the DC interrupt rating may be much lower than the AC rating.

Finally, the circuit breaker should not see routine use as a switch. Mechanical wear on the mechanism makes the trip point even more unreliable.

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Michael Griffin
London, Ont. Canada
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This is why you cannot use supplemental protection type CBs as banch circuit protection. However, a simple solution is to use a Square D QO style breaker with a din rail adapter. Much cheaper and will always work (up to its
capabilities). They do have the disadvantage of being only available in 10A sizes, but this is usually adequate. This can protect downstream
supplemental breakers.

OTOH-I much prefer fuses. But, they have the disadvantage of requiring spare parts, where as a CB can just be reset.

Bob Peterson

 

I have yet to see a circuit breaker that can beat a HRC fuse for short circuit protection. However, the HRC fuse is not much good on overload.
Consider this, the mechanical constraints of a circuit breaker considerably lengthen the time it takes for the circuit breaker to open on a short circuit as compared to a HRC fuse in which the element disintegrates very rapidly. You only have to look at the relevant let through energy curves for the 2 types of device and this becomes very obvious.
Each time a circuit breaker trips on overload or short the trip point shifts. The device has a thermal component which "ages" each time it heats up. A HRC fuse has a component of silver with a eutectic which either vaporises or does not.
Many years ago I worked for GEC, an English company that manufactured English Electric fuses. Some cartridges were removed from an old sub-station in London. These cartridges were installed at the turn of the century. They were dismantled and the silver elements had not suffered any deterioration or degradation in over 75 years. They still performed on short circuit test as originally designed. I do not believe a circuit breaker could possibly match that.
Horses for courses I guess.

 

My original question was about reasons for continuing to use fuses over circuit breakers when UL489 breakers are available in the form
I need (DIN rail, <10A). I guess I have some answers, but let me get some clarification.

Some specific example situations:
1. Panel programming receptacles which I typically fuse to 3A CC.
2. Small (<1kva) control transformers.
3. Panel fans / air conditioners / heaters.
4. Interior panel lighting.

Generally speaking, I would still be inclined to use J-TD fuses for main panel fused disconnects, 3-ph motors, VFD's, larger transformers, etc because of my personal comfort level. However, as
I understand it (please correct me if I'm wrong), a given panel's short-circuit interupt rating is that of its primary fuses / circuit breaker. If 50kA fault current is available and the panel's main fuses have a rating of 100kA then the fuses are suitable.

So, with such a panel, would it be OK, for example, to use a circuit breaker rated at 5kA for a control transformer??? In short (pardon
the pun), I don't have to use fuses for all the little stuff?

Anthony Kerstens P.Eng.

 

On April 25, 2003 12:29, Anthony Kerstens wrote: <clip>
> However, as
> I understand it (please correct me if I'm wrong), a given panel's
> short-circuit interupt rating is that of its primary fuses / circuit
> breaker. If 50kA fault current is available and the panel's main
> fuses have a rating of 100kA then the fuses are suitable.
>
> So, with such a panel, would it be OK, for example, to use a circuit
> breaker rated at 5kA for a control transformer??? In short (pardon
> the pun), I don't have to use fuses for all the little stuff?
<clip>

My understanding of the rule is that every element of the circuit must be capable of handling the short circuit current available at that point in the circuit. Unless the impendance of the system between the panel entry and the transformer were such as to limit the current, you would still need to meet the 50kA fault current at the primary of the transformer. A 5kA rated breaker may explode if required to interrupt a 50kA fault.

A motor overload may use back-up fuses to increase its rating, but the overload and fuse combination are designed and sized to accomodate this. This is not likely to be the case with the main fuses and the transformer primary breaker.

On the secondary side of a small transformer, the short circuit current availability is much lower, so short circuit interrupt capability is not normally a problem.

<clip>
> Some specific example situations:
> 1. Panel programming receptacles which I typically fuse to 3A CC.
> 2. Small (<1kva) control transformers.
> 3. Panel fans / air conditioners / heaters.
> 4. Interior panel lighting.
<clip>

For #1, 3, and 4, these are a classic examples of applications for a supplimentary protector. The wiring, receptacle, etc. are normally rated for 15 amps, and the available short circuit current is comparatively low. There is also normally a 15 amp branch circuit fuse which is consided to be the "real" protection. However, if *overload* protection is a concern, check the trip curves of the breaker. The standard rules of thumb may not be applicable for very small breakers due to manufacturing tolerances. Fires have resulted from this problem.

For #2, see above.

I don't want to discourage you from using breakers, but please keep in mind that breakers and fuses do not have the same performance characteristics, and the discrepancy is greater for the smaller sizes.

--

************************
Michael Griffin
London, Ont. Canada
************************

 

The short circuit capacity of circuit breaker should be more than the short circuit current level at the point of application. ie. circuit breaker comfortably handles the fault level without damaging itself and continue to serve its purpose of breaking the power connection. Higher the capacity than fault level more the no of trippings it can handle. Trip values are less than the capacity and some breakers have settable trip currents.

 

ABB has some UL489 CBs that go down to 0.2A and are DIN rail mountable. See the S200U and S200UP series.

 

Anthony
I agree with most of the posts HRC fuses are better at handling a fault in a high power circuit. However in my experience control system commissioning fuses can be real frustrating, a fuse blows due to a short somewhere in the field wiring and you don't have a spare on hand. How much easier to reset the breaker. Fault currents in my field are usually quite small, often limited by the power supply and small gauge wiring. I like the AB units with two buttons, it's obvious when they trip.

Regards
Roy