make a smart choice for your electrical protectionINTERNATIONAL

How to ensure your electrical installation is safe and reliable?

The aim of this white paper is to assist consultants and end users in making the most appropriate choice of protection devices for low voltage equipment.

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Electrical installation protection: it is your responsibility

Electricity can killEvery year, around the world there are thousands of accidents at work involving electrical shock or burns, some of which are fatal.

Risks-assessments and maintenance are paramount to safetyRisks-assessments have to be done to identify the precautions that are necessary to work safely.To prevent danger all electrical installations and devices should be maintained, inspected and tested periodically by a competent person.They must be maintained in a safe working condition throughout their working life.

In addition to the cost of human life is the financial costBeyond the cost of human life is the financial cost to industry, equipment replacement and downtime.A safe and reliable electrical installation, with adequate risk assessment is not only a moral obligation, but a financial as well as a legal responsibility.It’s our and your responsibility to recommend the best solution for a high level of safety and security.

Re-energising a circuit is a major riskRe-energising a circuit de-energised by a circuit protective device, is a major risk and should NOT be done by a simple resetting of a circuit breaker or replacement of fuses until it has been determined that the equipment and circuit can be safely re-energised.In any case, with fuses or with circuit breakers the person carrying out the work should have received adequate training.

Two options: fuses or circuit breaker .......................................................................................p. 3Fuse technology .......................................................................................................................p. 4Circuit breaker technology .......................................................................................................p. 5The right technology for the right application (low voltage only) ..............................................p. 6How to restrict the consequences of a fault: discrimination and cascading ...........................p. 7Maintenance is essential in order to avoid potential impact on workers and installations.......p. 8A high short-circuit limitation is essential for efficient protection of an electrical installation ...p. 9Economic cable sizing .............................................................................................................p. 10Glossary ...................................................................................................................................p. 11

SUMMARY

This image, taken from high-speed digital video shot during testing illustrates the type of blast than can occur in case of misapplication or misuse of low voltage electrical equipment.

For more information:bit.ly/profuse-arc_en

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Two options:fuses or circuit breaker

There is still an on-going debate whether the fuse or circuit breaker is the most suitable solution for circuit protection. Both devices work well and there are many applications in which either or both approaches are suitable.

Fuse protection: a solution which has proved itselfFor more than a century, the electrical fuse has been used as a protective means against overcurrents. today’s fuse-links have very high interrupting and current-limiting capability and are frequently applied in distribution and industrial networks as protection against overload and short-circuit currents.

Circuit breaker, another possibilityIn the early 1950s an alternative protection device became available with the development of the circuit breaker.

The wheel is turningThe trend of replacing fuses by circuit breakers has slowed down and fuse protection in many cases has returned as the preferred solution, especially in heavy industry, motor protection, DC current applications…For example most of the utilities in the world use fuses to protect their distribution networks.

Both solutions work wellBoth types of technologies work well in most of the applications and making a choice between these two alternatives requires proper understanding of an electrical installation and characteristics of the protection device. Its basic function is to interrupt current flow after protective relays detect a fault. Afterwards, a circuit breaker can be reset to reestablish the electrical circuit. The right choice will improve the safety and reliability of the electrical installation

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Fuse technology

A fuse consists of a metal strip fuse element, of smaller cross-section than the circuit conductors, mounted between a pair of electrical terminals, and enclosed in a non-combustible housing. The resistance of the element generates heat due to the current flow. The size and construction of the element are determined so that the heat produced for a normal load current does not cause the element to attain a high temperature.In overcurrent conditions, the internal temperature of the fuse elevates; fuse thermal dissipation capacity is lower than the thermal energy generated by the overcurrent) and a fuse element either directly melts, or else melts a soldered joint within the fuse, thus opening the circuit.

Whether the fuse opens or how fast it takes to open is dependent on the duration and intesity of the overcurrent.In case of a short-circuit the internal fuse element will melt/vaporise only at a specific amount of energy absorbed. This amount of energy is well below the total amount of energy potentially available under the faulted condition. The speed of protection provided by fuses offers, in most of the cases, the best short-circuit current limitation and thus provides a complete protective solution for all potential fault levels.

Fuse operation is based on a physical rule

Fault managementFuses must be replaced after a fault unlike a circuit breaker which can be reset.Nevertheless when new fuses are in place the performance of the protective device is restored to its original value.In addition the replacement is very easy.

OverloadThe internal temperature elevates, the calibrated fuse element melts.

Short-circuitThe restricted of the element vaporise and arcing commences.

The energy released is absorbed by the granulated quartz inside its body. It is contained in the sealed fuse cartridge.

Time

Current

125 A fuse-link

This curve represents the relationship between current and melting time of the fuse-link. The time current curves are defined by the applicable standards.

Replace the fuses and retain 100% of protection efficiency and safety

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Circuit breaker technology

A circuit breaker is a connection device able to make and break a circuit irrespective of current, up to its ultimate breaking capacity: Icu (refer to standard IEC 60947-2). Although its main function is to interrupt short-circuit and overload currents in case of overcurrent, it also breaks “normal” load currents by manual switching.

Three types of circuit breaker, basic principles1. MCB, Miniature Circuit Breaker•Ratings up to 125 A•Breaking capacity usually below 10 kA• For domestic and light commercial

applications, close to the load.

2. MCCB, Molded Case Circuit Breaker•Ratings generally from 100 to 1600 A•Breaking capacity from 36 to 150 kA• For power distribution, cables protection.

3. ACB, Air Circuit Breaker•Rating from 800 to 6000 A•Breaking capacity up to 150 kA• Incoming of main switchboard

They must remain closed in event of short-circuits and let the downstream circuit breaker eliminate the faults. Their operation is normally time delayed.

•Contact resistanceThe contact resistance must be as low as possible in order to reduce the heat generated at the contact point. The temperature rises can accentuate oxidation phenomena.On high currents, the heat produced at the contact point may exceed the heat which can be dissipated. In order to avoid melting and to prevent contacts welding specific material has to be used (silver alloy…).

•Contact repulsionMagnetic interaction between the current lines in the moving and fixed contacts leads to a contact repulsion force (at about 30 times the nominal current). The opening of a contact generates an arc, the arcing voltage will depend on the length of the arc. Contacts are affected by their inertia, to open fast the contacts must be as light as possible, and the repulsion force, the short-circuit high.

•Breaking the Short-circuitWhen electrical contacts open to interrupt a large current, an arc formes between the opened contacts, which would allow the current to continue to flow. This condition can create conductive ionized gases and molten or vaporized metal, which can cause further continuation of the arc, or creation of additional short circuits. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc in the most efficient and fastest manner. The maximum short-circuit current that a breaker can interrupt is determined by testing.

The trip unitsThe thermal-magnetic trip units:• In overload conditions, the tripping is achieved by a

thermal mechanical device, generally a bi-metal strip.• In short-circuit conditions, tripping is performed by a

magnetic circuit which actuates an armature or a core.

Electronic trip units:• Their prime purpose is evaluation of the current

flowing through the circuit breaker poles to take the appropriate action on a tripping device.

• Their main advantages are:- greater precision of target thresholds,- tripping curves which can be adjusted according to use.

Thermal protection

Electro-magneticprotection

Time

Current

MCCB 125 A

Typical operating curve of a thermal-magnetic circuit breaker.

How many times can a circuit breaker be reclosed?With respect to circuit breakers, itshould be considered whether the inspection of the device is necessary aftera fault clearance or how many fault currents he is able to interrupt without maintenance.

Increasing emissions due to degradation of the device after several trippings on short-circuit.

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The right technology for the right application (low voltage only)

Electricity availability has important consequences on companies’ competitiveness. Installations must be designed to limit the occurrence and consequences of failures. Power outage causes production losses, utility shutdowns (lighting, heating, lifts…).

Power availability is a key issueA reliable system is above all a quality system with reduced possibility of failure.Quality of design, characteristics and components (protection devices…) are of key importance in any system to guarantee installation safety and operation.

In power distribution, fuses are the best solutionIn a switchboard or panelboard a protective device operates in case of a fault (damaged cables, short-circuit after maintenance…) orlack of discrimination. The installation must be checked and correctly repaired. In this type of installation the short-circuit current may be high and reclosing on a fault may cause severe damages. For this type of installation fuses’ high breaking capacity and their unrivalled reliability make them the ideal circuit protection solution.

On the load side MCB’s are preferableA circuit breaker can be reset easily after a fault, this has benefits in domestic applications or where close to the loads, either in final distribution where electrical faults frequently occure due to overload conditions (too many heaters, boilers…) or due to the failures of the portable equipment connected through the power outlets (sockets)… At that level of the installation MCBs (MiniatureCircuit Breakers) are convenient and are more practical.

Power electronic converter equipment protectionToday’s industrial plants are using more and more soft-starters and drives. There can be a lot of confusion over how to best protect such sensitive devices. High speed fuses are the only products providing adequate protection of electronic motor controllers. Neither standard fuses nor circuit breakers can adequately protect the semi-conductors within these devices.

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High speed fuseFuse

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How to restrict the consequences of a fault: discrimination and cascading

The purpose of discrimination is to restrict the fault to the part of the installation where it has occurred. If discrimination studies are not correctly done, an electrical fault may cause several protective devices to operate, causing interruptions in the supply of power to other parts of the network.

Fuses provide total discrimination independent of the short-circuitIn case of high short-circuit the upstream (D1) and downstream (D2) breakers may both operate. That is the case when the thresholds of the instantaneous opening of the upstream (ins1) and downstream (ins2) breaker are too close.When discrimination exists only up to a certain short-circuit value it is called PARTIAL discrimination. With Circuit Breakers. With circuit breakers, a short-circuit current study is necessary to determine if discrimination is achieved for each circuit. With fuses total discrimination is simply achieved when the ratio of 1.6 is respected between the upstream and downstream fuses, independent of the short-circuit current.Fuses open the circuit when they “see” a specific level of current passing through the fuse. Lower amperage-rated fuses require less energy to open the circuit than higher amperage-rated devices. this allows TOTAL discrimination.

Discrimination is not affected by thebrand of the fusesThe fuse blowing and limitation curves arespecifiedaccordingtotheIEC 60269standard. This means that the brand of fuses does not have an effect on the discrimination. It is possible to replace a fuse with another brand without losing the discrimination.With circuit breakers it can only be guaranteed between products of the same manufacturer and the same generation.• Fuses:

•Circuit Breaker:

Circuit breaker: the risks of time discriminationA way to improve the discrimination between two circuit breakers is to delay the opening of the upstream circuit breaker (D1). this solution results in a total breaking time of the upstream circuit breaker generally greater than 50 ms. In this case there is no limitation of the current.If a fault occurs, the installation between the upstream and downstream protection device will support the full short-circuit current, and thus high thermal and electrodynamics stresses.• Time discrimination:

With fuses there is no need to allow for varying breaking capacities of protective devices. In some cases it is allowed to use a protective device such as a circuit breaker with a lower short-circuit rating than the maximum short- circuit current of the circuit. In such cases the upstream circuit breaker must have a higher short-circuit rating and be selected to protect the downstream device. This method of circuit protection allows for significant cost saving over using fully rated circuit breakers but carries significant risk of misapplication of the circuit breakers.By using fuses the risk is completely eliminated and the cost is often much lower than the circuit breaker alternative. Since fuses are all fully rated regardless of their current rating there is no need for concern over lower short circuit rated devices. Typically BS88 fuses are all 80KA, IEC fuses are 120 KA and UL fuses as high as 200 KA regardless.In summary, a fuse is the cost effective low risk alternative.

160 A100 A

Time (s)

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The fuse blowing curves are specified according to the IEC 60269.

Mini x 1.6

Total discrimination is achieved when the ratio of 1.6 is adhered between upstream and downstream fuses.

1.6 ratio is a guide only and does not guarantee discrimation.

Time (s)

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Limit of discrimation = Partial

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Total discrimination is difficult to achieve in case of short-circuit, often only partial discrimination is possible.

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The installation between the upstream and downstream circuit breaker must withstand

the full short-circuit current.

Isc

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D2Cascading: upstream circuit breaker (D1) protects D2which has a lower breaking capacity than short-circuitcurrent, in that case discrimination is not possible.

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Maintenance is essential in order to avoid potential impact on workers and installations

Safety in the workplace concerns us all and, in recent years, its importance has been emphasized by the introduction of a whole array of safety legislations. To prevent danger all electrical installations and devices should be maintained, inspected and tested at the frequency appropriate to the equipment by competent & authorised persons. An effective service plan will also reduce the shutdowns.

Circuit breakers should be periodically inspectedCircuit breakers are electro-mechanical products and should be periodically inspected and tested as part of the prescribed maintenance procedures. If circuit breakers are inadequately maintained and tested, their operating and interrupting capability, ability to protect system components, and electrical safety of the system may be compromised.

Fuses are maintenance free devicesA fuse is a static protection and is a totally sealed product. In the event of an over-current the fuse will melt first and thus protect the other components of the circuit. That guarantees long-term protection in the most arduous environmental conditions without maintenance. The fuse element reacts to a physical rule according to the energy, and in event of a fault it will always operate.

The energy released during a short-circuit is contained within the sealed fuse cartridge but not in a circuit breakerWhen operating under a short-circuit condition, as the contacts of a circuit breaker separate, an arc is created between the contacts. The resultant expansion of ionized gases must be vented “safely” from the breaker into the surrounding equipment.

Can you guarantee the electrical characteristics after a short-circuit?A circuit breaker can be reclosed after a fault while the fuse has to be replaced. However, when fuses are replaced after fault clearance, the installation is as reliable as before the short-circuit occurred.How are the circuit breaker contacts after a fault? How many times has it operated and at what fault level before an operator will close it?

Irrespective of its age a fuse will always operateWith regard to the life time of the protective devices, aging of fuses and circuit breakers. Irrespective of its age a fuse will always operate. The risk of failure on mechanical devices such as circuit breakers increases with age and so a process of assessment in order to decide the appropriate action is needed.

The energy released during a short-circuit is contained within the sealed fuse cartridge.

The hot ionized gases can potentially damage other components or persons.

Ageing affects electro-mechanical devices.Fuses are “static” components and their

performances are not affected by the surrounding environment or age.

Reliability

Time

Circuit breakerFuse

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A high short-circuit current limitation is essential for efficient protection of an electrical installation

A short-circuit current can be many hundreds of times larger than normal operating current. If not cut off in a matter of few milliseconds, there can be severe installation damage, melting of metals, ionisation of gases, arcing and fires. Simultaneously there can be huge magnetic-field stresses to cables and equipment.

Fuses: the best solution for short-circuit limitationFuses and circuit breakers protect against short-circuits.One of the main advantages of a fuse is its high limitation of the fault current and energy. When a fuse link interrupts a high value of fault current itlimitsthecurrentbeforeithastimetoreachitsfullvalue.Current andenergy limiting are important features of a fuse, it ensures that the electrical equipment, (cables, motors, etc.) are protected against the electro-dynamical force and heat generated during a short-circuit.

All industrial fuses have at least 80 kA breaking capacityA protective device must be able to withstand the destructive energy of short-circuit currents. If the fault current exceeds a level beyond the capacity of the protective device, the device may actually rupture, causing additional damage. All fuses IEC or DIN have a 100 kA breaking capacity (sometimes 120 kA). BS 80 kA,ULfuseshavebreakingcapacityupto200kA.Itispossibletofindcircuitbreakers with very high breaking capacity, the more recent products are current limiting. Nevertheless it is necessary to choose the right product as breaking capacity varies from one product to the other, from 3 kA to more than 100 kA.A circuit breaker with similar breaking capacity to a fuse will be physically much larger than its equivalent fuse.

More energy there is, the faster the fuse operatesThe breaking time of a fuse depends on a physical rule, the more energy there is, the faster the fuse operates. A circuit breaker is a mechanical device and the opening time is affected by the inertia of the tripping mechanism, that is why the operating time of a circuit breaker decreases only slightly when the short-circuit current increases. Some circuit breakers use the energy generated during the fault to increase the speed of operation but that is to the detriment of the breaking characteristics at more common low fault levels.

The breaking capacity of a circuit breaker is affected by the voltageThe breaking capacity of most of the circuit breakers is dramatically reduced when the voltage increases, often the breaking capacity at 690 V is less than 30% of the value at 400 V. The breaking capacity of breakers can decrease from 100 ka at 400 V to 20 ka or even less at 690 V.Thebreakingcapacityofafuseisnotaffectedbythevoltage.The breaking capacity of protective devices are affected by the voltage.

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The short-circuit is eliminated in less than 5 ms.

Current kA

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The breaking capacity of protective devices are affected by the voltage.

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The higher the fault current, the faster the opening

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Economic cable sizing

In the current economic climate financial decision makers need to be aware of every cost saving opportunity. Paring of capital budget by buying the cheapest possible equipment can result in very high running costs throughout the lifetime of the equipment.

Fuses protect cables against overloadLow voltage gG type fuses protect cables against overload and short-circuit. The rules for the selection of an overcurrent protection of cables are defined by the international wiring standardIEC 60364,standardforelectricalinstallations.

The smaller the cables are, the higher the watt losses areUntil the recent worldwide economic downturn, it was common practice to install cables that were two standard sizes larger than the minimum recommended simply in order to allow for increased demand. There was an immediate benefit from a reduction in I2R losses and long, reliable lifetimes for the cables due to their running at less than their maximum rated temperature.•Economical cable size

Minimum conductor sizes approach is not the most economicalMandatory regulations specify minimum conductor sizes for thermal safety but are not intended to be the most economical if energy losses throughout the life of the power cable are taken into consideration.Now, new costing policies may demand lowest first cost by installing the minimum permitted cable size and result in these benefits being lost. Circuit breakers often allows the use of smaller cable than fuses do.

Additional costsThe cost of excess heat removal can more than double the cost of I2R losses in cable ducts. In modern office and industrial buildings with a heavy demand for power, air conditioning plant has to be used to export surplus heat for much of the year, this cost must be taken into account.

Installing a larger cable than the minimum requested size can return quick paybackInstalling cables only one size larger than the minimum requested by the regulation increases energy efficiency. This simple technique can yield quick paybacks. By upsising cables in a new installation, the engineer or contractor can demonstrate the real savings to the customer as well as the advantages of lower generated heat, lower voltage drop and increased flexibility of the installation.Of course, there are many factors that must be considered in any installation. But for most new applications, where the cost of labour and conduit for the installation outweigh the cost of cables, the increased size of the cables can pay for itself in less than two years.

Cost savingFor example, a cable of 16 mm2 section can be rated at 109 ampsandrunat90°Cbutovera100metrerunwoulddrop over 30 volts and waste nearly 3.5 kW. this would cost over1.500£peryearifelectricityisboughtat0.05 £perunit.the use of 35 mm2 section more than halves this wastage.

Fuse time/current curves are adapted to cable time/current curves.

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Fuse time/current curves are adapted to cable time/current curves.

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Waste electricity can shorten equipment life.

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Circuit breaker definitions

Breaking capacity, Icu and Ics (IEC 60947-2)

• Icu (Rated ultimate breaking capacity) Icu (kA RMS) is the maximum short-circuit current value that the circuit breaker can break. It is verified according to a sequence of standardised tests O-C-O (Open-Close-Open). After this sequence, the circuit breaker must not be dangerous. This characteristic is defined for a specific voltage rating Ue.

• Ics (Rated service breaking capacity) lcs (kA RMS) is given by the manufacturer and is expressed as a% of lcu. This performance is very important as it gives the ability of a circuit breaker to provide totally normal operation once it has broken this short-circuit current three times.

GLOSSARY

• Prospective short-circuit current: Possible Short circuit currents when there is no short-circuit limitation device.

• Breaking capacity: This is the prospective short-circuit current value that the fuse can interrupt under a specified operational voltage, typically: 120 kA (IEC) & 80 kA (BS).

• Fuse let through current: This is the real value of current flowing through the fuse when a fast current limiting device reduces the short-circuit current.

• Pre-arcing time (0 - t1): This is the time necessary for the current to bring the fuse element to vaporisation point before melting. Pre-arcing time is independent from the network voltage.

• Arcing time (t1 - t2): This is defined as the period between the instant of arc appearance and its total extinction (zero current). Arcing time is dependent from the voltage.

• Operation time (0 - t2): This is the sum of pre-arcing and arcing times.

• HRC: High Rupturing Capacity – the ability of a fuse link to interrupt high fault currents

• Rated Voltage: The maximum voltage the fuse link is designed to interrupt

• Current Rating: The value of current a fuse link will carry continuously without deterioration.

• NFC fuses:- worldwide, except in USA,

UK and Commonwealth,- sizes: 10x38, 14x51

and 22x58 mm- low ratings (up to 125A),- with or without strikers,- elastic connection.

•DIN fuses- worldwide, except in USA,

UK and Commonwealth- sizes: 000 to 4,- intermediate ratings

(up to 1600A),- with or without strikers,- elastic connection.

•BS fuses- mainly UK and

Commonwealth,- sizes: F1 to D1,- all ratings (up to 1250 A),- no strikers,- bolted connection.

•UL fuses- mainly USA,- all ratings (up to 6000 A),- no strikers,- elastic or bolted

connection.

Fuse technology definitions

Main fuse types (per area)

kA

ms

Circuit breaker

Let-through current

High speed fuseFuse

Which standards?• Fuses

- 60269-1 (generic fuses)- 60269-2 (other fuses)- 60269-4 (high speed fuses)- 60269-5 (application guide)- 60269-6 (PV fuses)

•Circuit breakers- 60947-1- 60947-2

make a smart choice for your electrical protectionINTERNATIONAL

Pro Fuse International is a global association established to inform, educate and guide the industry in the vital role played by the fuse as the

leading method of protecting electrical circuits.

Who is Pro Fuse International?

LEARN MORE More about safe and reliable electrical protection?www.profuseinternational.com

Did you knowfuses are usedin the followingapplications?

Rail

Electric Vehicle

Solar

Smartphone Racing Cars

Utility

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