Why MCBs are the Preferred Choice Over Fuses: An Overview

For many years the fuse was the best choice to protect man and machine from overcurrent. However, what moved into the consumer units of private households over three decades ago, is now also starting to increasingly establish itself – in a more developed form – in industry, business, and transport: the miniature circuit breaker.

Miniature Circuit Breaker

The miniature circuit breaker (MCB) has been in existence since the 1920s. It is characterized mainly by the feature that it can be switched on again after it has tripped, by simply switching a lever by hand without using any tools. Furthermore, it offers consistent selectivity and tripping characteristics over the whole operating period and therefore constant reliability.

Office complexes, airports, solar parks, high-speed trains: there’s a good reason why this well-thought-through and robust solution for the protection of power lines is used increasingly where maximum safety and the highest availability are of paramount importance.

Fuse

The classic amongst current overload protection devices. A fuse offers high levels of switching capacity and good current limiting. Apart from having good selectivity features when new, they are markedly cheap when initially used to provide fuse links. Add to this the fuse’s most significant characteristic: once it has tripped it must be completely exchanged for a new one.

Why MCBs Are Better Than Fuses?

MCBs are preferable to electric fuses for the following reasons:

1. A miniature circuit breaker can be tested reliably

It is the nature of the thing: when a fuse is produced only random tests can be carried out and tripping efficiency checked. A function test is impossible, as this would destroy the fuse. In practice, we can rely only on a spot test when checking the fuse for statistical protection.

However, those who want to check the quality of an overcurrent protective device not only by means of optical inspection, simple continuity tests, or shaking, rely correctly on miniature circuit breakers: These are always tested pole to pole in the factory and are calibrated. High-performance MCBs can be checked at any time via the test button, even when built-in. This offers maximum safety. Indeed, at any time.

2. MCB guarantees excellent operator safety

Most accidents involving electricity do not have a technical cause! Aluminum foil wrapped around burnt-out fuses, a finger in the fuse socket, the pulling of fuses by force without safety equipment, a discharging metal wristwatch, or the common mentality “let’s just leave it“– all contribute to the findings of the Institute for the Investigation of Electrical Accidents in the Trade Associations Energy, Textiles, Electrics, and Media Products, that around 85% of all reported accidents involving electricity even happen to qualified electrical personnel.

Therefore, electrical accidents happen especially often in low-voltage distribution. It is not surprising that arc accidents are named as the third significant cause for electrical accidents considering the recently popular use of fuses in photovoltaic installations. In particular, the high levels of DC current in photovoltaic installations can create dangerous arcs when fuses are inexpertly changed without first operating the disconnector. And this happens despite the fact that miniature circuit breakers for the safety of wiring for photovoltaic installations have been available for several years, which can remove this safety risk with a click once and for all.

Another benefit of the miniature circuit breaker: It eliminates the risk that a fuse with the wrong current rating is installed

– which is quite possible in the fuse sockets of generator junction boxes.

It is a fact that safety guidelines are not always observed when fuses are changed and that safety equipment is often avoided. Furthermore, the everyday practical exchange of fuses always carries a risk – which simply just does not exist when you comfortably switch the toggle of a miniature circuit breaker to remedy a fault.

3. Disconnector properties of MCB combined with safety

A burnt-out fuse cannot disconnect. A disconnector must be fitted in front when it is not already integrated into the fuse socket. This takes up more space on the DIN-rail than a miniature circuit breaker with disconnection features – an appliance that can do both: protecting and disconnecting.

4. Fuses change their properties but MCBs do not

This is often neglected with fuses: they are subject to an aging process. Levels of efficiency with regards to selectivity or Back-Up can only be determined at one point and always only refer to a new fuse

– as power surges during its lifetime can change the fuse’s ability to react. A fuse is unreliable, as the fusing process determines the operating time of disconnection. It has been reported that even sudden changes in temperature without overload or short circuits can cause the fuse to blow.

3. Constant efficiency over MCB’s lifetime

A miniature circuit breaker guarantees years of consistent efficiency. Each pole is secured individually; in case of faults, it switches all pols off and all trigger features remain reliable throughout. Over its entire lifetime.

4. Environment friendliness meets cost-efficiency

Burnt-out fuses must be disposed of. A triggered miniature circuit breaker can be switched on again, comfortably and safely, when it has tripped – even by remote control, which is invaluable especially for solar power stations and wind farms.

Another plus for the environment is that circuit breaker components can be easily recycled, and that the miniature circuit breaker saves space.

5. Faster operating, avoiding stoppages and long journeys

A burnt-out fuse must be replaced. You are forced to store an appropriate replacement. If you are lucky, you can purchase it, if you are unlucky, it is not easily available in the country where the unit is being operated and therefore causes unacceptable delays.

Furthermore, appropriate safety equipment must be available during the exchange which, above all, has to be available. This causes further hold-ups in production in these cases.

It is often difficult to ascertain that a fuse has burnt out. And a burnt-out fuse generally has no remote-control tripping signal. This, however, can be easily realized when you use a miniature circuit breaker by installing an auxiliary or signal contact; its switch position is always clear.

Modern miniature circuit breakers have a ‘TRIP’ position. Resetting the miniature circuit breaker is also possible by hand without any tools. Modern equipment series also offer remote control units.

6. Low maintenance costs determine the favorable start-up price

This is a statement often made by electrical planners:

“Installations without fuses are a problem of investment. If the customer does not especially demand a miniature circuit breaker, my competitor will always be cheaper. Everyone wants safety but does not want to pay more for it. “

The often-low initial cost of fuses may make this statement sound correct, at first. However, often neither the cost of the fuse box nor the continuous on-costs during the lifetime of the equipment – the so-called Total Cost of Ownership (TCO) are considered. This does not just mean the costs for a certain number of spare fuses, transport, or personal safety equipment. The highest cost difference between miniature circuit breakers and fuses is the much lower downtime of the electrical appliances connected to the circuit. Profits lost add up to gigantic sums for companies, especially for commercial and industrial low-voltage circuit users. The Internet offers Online-Calculators to work out profits lost due to downtime. Non-available mobile telephone stations cause losses of profit of over 600 EUR per minute, a stock exchange system that does not function deficits the accounts by more than 120‘000 EUR per minute. Costs, which in the first place don’t even occur when an MCB has tripped and its investment cost soon balances out. Circuit protection management with miniature circuit breakers ensures that even in times of low charge returns in a photovoltaic installation the greater cost compared to fuses can be written off within the first hour of the first fault.

The operator of an installation

The operator increases his profits, due to minimized downtimes and a safe installation, with low recurring costs. This allows him to achieve competitive advantages and he can apply his resources to investments, which add value.

The planner or designer of an installation

The planner or designer of an installation advises and consults with good arguments for the miniature circuit breaker – and makes an impact as a partner when recommending miniature circuit breakers.

He secures further competitive advantages and follow-up commissions from satisfied customers. Furthermore, daily work is simplified in as much that the size of the safety switch always remains the same independent of power supply and voltage.

7. MCBs are easy to stock and sell

The uniform, compact size simplifies storage and decreases the otherwise necessary product range. The higher initial costs of the miniature circuit breaker also guarantee turnover.

Source: Day by Din – ABB