Circuit Breakers For Self Preservation

Selection of Eaton Memshield MCB circuit breakers

Following on in the same theme as my earlier post Are You Protected - Power Surges, we discuss in more detail the various circuit breakers that can be used in a simulator setting. Additionally, it is stressed that electricity can kill and a qualified electrician should be contacted prior to implementing anything suggested below.

My thanks to Dave Egkkman (flight simulator enthusiast based in the United Kingdom) who has written this article. I think the content adds considerably to my earlier post and I am grateful to Dave for writing and allowing it to be posted here.

Circuit Breakers and Fuses

It is a common misconception that fuses and circuit breakers are there for personnel protection. It is very important to understand that this is not the case.

It can take as little as 0.03A to kill a human being, that’s 30mA! So, if you have a circuit protected by a 30A fuse/circuit breaker, it will allow 1000 times more current to flow than it takes to kill, before it breaks the circuit and stops the current flow.

Fuses and circuit breakers (protective devices) are there to protect the cabling in an electrical circuit from damage by overload. This is achieved by installing a suitably rated circuit breaker at the beginning of a circuit. The rating of the protective device will be calculated to protect the lowest rated cable in the circuit, by stopping the flow of current in an overload situation before the current flow exceeds the current rating of the cable.

A common scenario in the UK is the use of 13A fuses in plugs. In domestic plugs in UK, the plug can take a variety of fuses rated from 1A to 13A. Often loose plugs are supplied fitted with a 13A fuse or, if a fuse ‘blows’ the user will commonly fit a 13A fuse as a replacement. However, if the appliance is a lamp standard, the cable from the plug to the electrical appliance could be rated at 6A or less.

So, in the event of a fault developing that allows 10A to flow, the fuse will not break the circuit, but the cable could well overheat and catch fire, and still the fuse may not break the circuit.

Fuses

A fuse is simply a piece of small wire designed to melt and break the circuit if more current flows than the fuse wire is designed to allow to flow.

In some cases, faults can occur in which the current may not be large enough to melt the fuse but enough to seriously harm the user of the electrical appliance. Circuit breakers generally offer better protection

Circuit Breakers

For domestic installations there are three main types of circuit-breakers:

Miniature Circuit Breaker (MCB);
Residual Current Circuit Breaker (RCCB) or Residual Current Device (RCD); and,
Residual Circuit Breaker with overload (RCBO).

Protec 63A 363-2C-6kA Triple Pole MCB circuit breaker

Miniature Circuit Breaker (MCB)

The MCB is a protective device typically used where a fuse would have been. MCBs are quicker to respond than fuses, are more reliable, more sensitive and can be reset once the fault has been cleared, the problem identified and fixed.

There are many different configurations of MCBs, which we won’t go into here. We should however consider the three different ‘Types’ that are available. All 3 MCB types use a magnetic fault protection, which trips the MCB within one tenth of a second when the overload reaches a set level.

How a MCB Works

Basically, the live input cable is wound around an iron core. As current flows a magnetic Field is generated. If the magnetic field reaches a pre-set level, an iron latch is pulled towards the iron core (magnet) and breaks the circuit.

In normal domestic use a Type B MCB will be used. A Type B breaker will trip between 3 and 5 times full load current.

For electrical loads that have a high inrush current when they are switched on, a Type B breaker is likely to trip as it ‘sees’ the inrush current as an overload.

A Type C breaker trips between 5 and 10 times full load current. This allows the MCB to withstand the initial inrush current, whilst still providing overload protection in normal use.

A Type D breaker trips between 10 and 20 times full load current, typically used where high inductive loads are present such as motors and transformers.

For those with problems of an MCB (or a fuse) tripping when turning on equipment, a Type C breaker may be the answer. Selecting the correct Type and rating of breaker is not an arbitrary decision. An electrically qualified person should make this decision.

Whichever MCB type is used, it is extremely dangerous to cut corners by using inferior quality devices, therefore they should only be bought from a reputable supplier. Copy and cheap MCBs have been found to have no copper/nickel/silver contacts within them, just steel. Upon introducing a fault, the contacts simply weld together, the consequences are obvious. Don’t go cheap.

ETI 25A 30mA RCD 2 throw circuit breaker

Residual Current Device

A residual current device IS designed to offer personnel protection. RCDs are used in combination with fuses and MCBs.

Residual current circuit breakers work by comparing the current entering the appliance via the live input with the current leaving the appliance through the neutral.

How a RCD Works

The live wire and neutral wire within the device, are wound around iron cores in opposite directions. When the appliance is working correctly ALL the electrical current entering the appliance via the live wire, exits the appliance through the neutral wire. The magnetic fields generated around the iron cores cancel out.

In the event of a fault some of the electric current will flow through the earth wire, casing of the appliance or in the absence of proper earthing through the body of the user. This results in an imbalance between the current entering the appliance through the live wire and the current exiting through the neutral wire.

This difference in electrical current is called the residual current and it is what causes the device to break the circuit.

Residual Current Circuit Breakers have the advantage of being highly sensitive with a very quick response time.

There are various ratings of RCDs. Typically, in domestic use a 30mA RCD will be used, but 10mA is also common. Selecting the correct Type and rating of RCD is not an arbitrary decision. A qualified person should make this decision.

It is not unusual for people to complain that RCDs suffer from nuisance tripping. If an RCD is tripping there is a problem, the problem should be identified and corrected. If an upstream RCD is tripping, rather than the local RCD, for example the RCD in the house trips and the one in the out building does not, then the configuration of the circuit is incorrect. These issues should be addressed by, yes you guessed it, a suitably electrically qualified person. Issues of disconnection times, voltage drop, resistance of the earth path all need to be considered.

Residual Circuit Breaker with overload

A residual circuit breaker with overload (RCBO) protection is a device that combines overload and personnel protection.

They are often used where there is not enough space for an MCB and RCD in one consumer unit / fuse board.

Disclaimer

This information is provided to offer guidance only and hopefully to suggest when an electrically qualified person should be approached for guidance. It is not comprehensive and only scrapes the surface of the subject of electrical protection.

I don’t want to come across as ‘holier than thou’ but, I really don't wish to be drawn into offering guidance to people about which and what protective devices should be used for their particular installation. I don't even agree with DIY companies selling fuse boards, MCBs etc. I'm hoping my words will encourage folks to seek professional advice from a local engineer/electrician who can assess their own situation. It's not the same as deciding which interface card to use for TQ servo activation!

All electrical work must be carried out by a qualified engineer/electrician and this post is not suggesting otherwise.

Glossary

A - Amp

mA - Milliamp