Fuse Elements

The S&C 11 kV Cut Out expulsion fuse links for 11 kV Cut Out Fuses are designed to provide an improvement in power distribution system reliability via reduced fault incidence and lower overall costs. S&C manufacture Positrol Fuse Links in six different speeds, and Electropar stocks Standard Speed (1 through 5amp), T Speed Fuse Links (6 through 10Oamp) and K Speed Fuse Links from (6 through 30amp). S&C manufacture only standard speed in the 1 -5 amp range. Reason: the TCC curves are so close, that T, K or E can't be differentiated.
The S&C Positrol 11 kV Cut Out Fuse Links offer some unique performance features as follows:

Silver Fuse Element
Positrol fuse links are unique in the respect that they have a silver fuse element rather than an element manufactured from tinned copper. S&C are the only manufacturer to offer silver elemented 11 kV expulsion fuse links.
The fuse element is the key to a Cut Out fuse's performance. The silver element in the Positrol link has a 900°C melting point, compared to 250°C for a tinned copper element. Additionally, the tinned copper link has a 20% confidence interval either side of its nominal current carrying capacity whereas the silver link has a 10% confidence interval. This means the silver link will operate in a much more defined band from the time the fuse element starts to the time the short circuit fault is cleared. Technically, this is important as it allows protection engineers to create tighter differential protection between network plant (ie. fuses, reclosers and ultimately substation circuit breakers) and have confidence that it is going to work as planned. Tighter protection means that in the event of a fault an absolute minimum number of customers can suffer unplanned outages, reducing the potential for charter claims and improving SAIDI and CAIDI performance. Clearly, this is a means of creating additional revenue for the asset owner.
Practically, the use of silver elemented 11kV drop out expulsion fuse links creates improved protection If a tinned copper 20amp fuse link sits at or near its rated current carrying capacity for a period of time it will become hot - close to the 250° melting point. Subsequently, a low magnitude fault can further raise the temperature of the element so the tinned copper begins to melt, but the fault is not sufficient to make the element clear. In effect, the fault has not been sufficient to jump the "high side" of the 20% confidence interval.
As the element cools without having cleared, the tinned copper fuses back together again but the characteristics of the element have changed as a result of the heat cycle. The fuse element could now be rated at perhaps only 10amps. Taking this hypothetical situation a step further, if one fine, sunny Saturday morning the damaged element is forced to perform close to peak load - it cannot and consequently clears for no apparent reason. The result is a costly fault call out.
The silver element in S&C Positrol fuse links does not begin to melt until 900cC. As such, Positrols are far less vulnerable to damage as a result of heat cycling. Additionally, the confidence interval between the fuse element beginning to melt and clearing is half that of a tinned copper element fuse link (10%). This means that the Positrol fuse link will reach its confidence interval quickly in a real fault incidence better protecting the expensive transformer asset. In effect, the Positrol is a "smart" fuse link.

S&C Positrol Fuse Links in Lightning Prone Areas
In areas that are prone to lightning, there are also distinct advantages in having a silver element. Lightning has a low level of total energy. It can cause heating of the fuse element in a micro-second without breaching the 20% confidence interval of a tinned copper fuse element. In effect, the lightning partially melts the fuse element but the speed of the heat cycle does not allow the tinned copper element to reach total clear. This situation is almost impossible to achieve with an S&C silver elemented 11 kV Drop Out expulsion Fuse link - the fuse links at the epicentre of the lightning strike will fail, but those on the periphery of the strike that do not drop out will not have had their fuse characteristics altered.

Positrol Fuse Links - Fibreglass Tube Sheath
S&C 11kV Cut Out Positrol fuse links are supplied as standard with a fibreglass tube protecting the fuse element. Fitted to double vented Drop Outs, (of which there are many still on NZ Power Company Systems) the fibreglass will not swell when in contact with water as the phenolic paper covered fuses do. S&C is the only manufacturer to offer an 11 kV Cut Out Expulsion Fuse Link with a fibreglass sheath that will not swell when in contact with water. Should a phenolic paper covered fuse element swell to a point where it sticks to the inside of the fuse tube, a fault can be prevented from clearing the Cut Out and the fuse tube can "hang up" effectively stressing or destroying any equipment the Cut Out is protecting.

Universal Fitment and Colour Coding
S&C Positrol fuse links are universal fitting all commonly used Drop Out fuses used in New Zealand. Positrol fuse links are supplied with a ferrule crimped onto the tail of the tinned copper braid to make for easy fitment to some of these. Each fuse link is supplied individually wrapped. The size of the Positrol fuse link is labelled on the packet and the fuse link itself is stamped with the amperage. Electropar colour codes the fuse links so that they replicate the fuse link colour codes used in New Zealand for many years.