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DC Unit - Element 7
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Terms associated with fuses
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Unit Standard 2
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This week's Objectives
High Rupturing Capacity (HRC) Fuse
The HRC fuse is a fully enclosed cartridge type fuse that has a high rupturing capacity. It is able to withstand and interrupt high current, short circuit faults safe
A cutaway of the construction of the HRC is shown below.
The fusing element is enclosed in a porcelain body surrounded by silica sand. The end caps have securing lugs. The element has constrictions and a tin alloy section.
Under normal operating conditions the current flowing through the fuse element does not provide enough energy to melt the element. The heat produced is absorbed by the surrounding silica sand.
If a large current flows the energy produced melts and vapourises the element.
The time taken to reach this point is known as the pre arcing time. The high prospective fault current is ‘cut off’ although an arc will still be formed. The heat produced by the arc causes the fuse element to fuse with the silica sand and for the silica sand to absorb the energy. This has the effect of extinguishing the arc and stopping further current flow. The time taken to reach this point is known as the total clearance time.
Modern HRC fuse elements incorporate a tin alloy section. This is known as a eutectic material. It is used to give the fuse specific operating characteristics. Under overload conditions the material heats up and if the overload is prolonged, it will melt and break the fuse element.
Short Circuit Condition
Under high current short circuit conditions the smaller area constricted parts of the element will melt rapidly and vaporise. These will break before the eutectic material.
The quartz silica sand surrounding the element is also heated and changes into a glass material called fugurite. This quenches the arc and prevents any restriking across the element gaps.
A high fault clearance graph is shown below
The pre-arcing time (shown by A) is the time given by fuse characteristic charts. The figures given by the graphs are reliable for all conditions. This is the time taken for the fault current to melt and break the element. However it is at this time that an arc will form and the current will continue to flow. The quenching qualities of the silica sand will extinguish the arc and stop the current flow. The time taken for this is less predictable (shown by B) as there are many factors to vary its duration.
The total operating time (or clearance time) is the combination of the two and is shown by C. The cut off current is shown by D and indicates the maximum fault current flowing in the circuit. Quick clearance of the fault ensures that the cut off current does not reach prospective fault current level.
Advantages of the HRC Fuse
The HRC fuse has many advantages over the rewirable fuse. These include:
The ability to safely interrupt short circuit currents of much higher values (higher rupturing capacity). They are specifically designed to operate under these short circuit conditions.
The elimination of arcing because the fuse element is sealed
It is obtainable in a range of fusing factors
The current rating is clearly marked
It has a reliable operation
It can be used to provide good discrimination because it provides close protection and operates at a ‘known’ value.
It has constant fusing characteristics
It operates ‘fast’
Doesn’t deteriorate over time
There have been occasions when ‘operators’ have bridged the terminals of a HRC fuse carrier with fuse wire of the same current rating as the blown fuse. Although this might seem safe there are many reasons why this should definitely NOT be done.
As we have seen before fuse wire (within the rewirable fuse) does not have the advantages of the HRC fuse and will not protect the circuit in the same way.
It is therefore NOT permitted to bridge the terminals of HRC fuses with fuse wire because:
If the fuse blows again an arc may be established between the fuse terminals causing damage or injury
It cannot safely interrupt high value short circuit currents
The fuse wire may extend past the holder and therefore create a chance of electric shock
Suitable fixing of the fuse wire is not generally available
The fuse holder is not fire proof
Slower operation under fault conditions
The fusing factor will be changed
Replacing a HRC Fuse
When replacing a HRC fuse the replacement fuse must have the same characteristics as the original. More specifically it must have the same:
Category of Duty (Rupturing capacity)
Utilisation category (fusing factor)
If the current rating of the replaced fuse is too low the circuit could blow (break) for no apparent reason well below the circuit full load. For example if a 5A fuse is put in a circuit designed for a 10A load the 5A is going to blow even under normal load conditions.
If the current rating is too large for the circuit and a fault occurs, the circuit current could increase to a high level causing damage, before the fuse operates. For instance if a 20A fuse was used to replace a blown 5A fuse and a fault in the circuit caused 15A to flow (which would ‘blow’ the normal 5A fuse) it would not operate the incorrect 20A fuse.
If a blown fuse is replaced in a circuit that is turned off (and all appliances are removed) and the fuse blows again the problem will be within the fixed wiring of the circuit. This should be reported to a Registered Electrician.
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