They are both electrical terms and have precise definitions. (See below)

Work out the level of earth fault current

A circuit will always have a certain amount of impedance or resistance. The lower this impedance is the more fault current will flow. For now though it is suitable to understand using Ohms law that the lower the impedance the higher the fault current.

Each and every device is manufactured with a breaking capacity. This means that it will be made to be able to withstand a certain level of fault current. If an over-current device is installed where its breaking capacity is less than the expected fault current it can have devastating effects:

The over-current device can explode.

It can melt and weld in the closed position.

The device can catch fire.

To establish the breaking capacity of an over-current device depends upon the device and we will look at each type on the next pages. Generally, it can be visibly seen on the device or manufacturer’s information may need to be consulted.

Let us consider the breaking capacity of an MCB is 3kA (3000 amps). In order for that device to withstand a level of fault current it must not exceed 3000 amps. The maximum resistance (or earth loop impedance as it is known) should not exceed 0.07 Ω.

For this reason maximum earth loop impedance values for each device is listed in the Tables from BS7671 and Onsite Guide. See table 41.2, 41.3 and 41.4.

Explain why would the device melting and welding in the closed position be dangerous?

Overload rating

BS7671 calls an overload:-

An over-current in a circuit that is electrically sound.

This seems a bit strange, after all what is the problem if the circuit is sound? A circuit may be sound, however the load or appliance connected to the circuit may demand more current than has been allowed for in the design.

Imagine connecting a fire, which demands 20A, to a circuit that has only been designed to carry 10A. The circuit is not at fault; it is the person who has connected too large a load to the circuit who is at fault. Therefore the circuit is sound and it is the introduction of an extra element that causes it. Motors can also overload when the mechanical demand is increased and the motor over compensates to keep to the same speed.

Here we see a socket that has had to supply more current that it is designed too for a long period of time.

So an over-current device is designed to carry an amount of current indefinitely. If this value is exceeded then the device should operate before a hazardous event occurs.

What can happen in the event of an overload?

Identifying these ratings

Semi-enclosed / re-wireable fuses BS3036

The over- current protection of these devices is achieved by fuse wire that can be changed by the user. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are colour coded to display their

supposed rating. Although as the fuse wire can be replaced by the user it has been known for 5 Amp devices containing larger 15 or 30 Amp fuse wire.

The main BS3036 fuses used today are shown below. They are identified by their colour.

5 Amps – White 15 Amps – Blue 20 Amps – Yellow 30 Amps – Red 45 Amps – Green

These devices are classified into three categories for short circuit rating. The ratings of these are not apparent and the manufacturers should be consulted.

S1A – 1KA (1000 amps) S2A – 2KA (2000 amps) S3A – 4KA (4000 amps)

Cartridge fuses to BS 1361, BS 1362

The over- current protection of these devices is achieved by the fuse wire that is contained within the glass body of the device. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are colour coded

The main BS1361 and 1362 fuses used today are shown below. They are identified by their size and their colour. The ratings however are apparent by observing the body where they are also written in amps.

3 Amps - Red 5 Amps – White 13 Amps - Brown 16 Amps – Blue 20 Amps – Yellow 30 Amps – Red 45 Amps – Green

These devices are classified into two categories for short circuit rating. The ratings of these are not always apparent and the manufacturers should be consulted.

Type 1 – 16.5KA (16,500 amps) Type 2 – 33.0KA (33,000 amps)

HBC Fuses (BS 88)

The over- current protection of these devices is achieved by the fuse wire that is contained within the glass body of the device. The thickness of the wire determines the amount of current it can safely carry. The bodies of the devices are not colour coded and vary from between manufacturers.

They are identified by their size the ratings are apparent by observing the body where they are written in amps.

These devices are classified into two categories for short circuit rating. The ratings of these are usually apparent by observing the body of the fuse.

BS88-2.1 – 50KA at 400V (50,000 amps) BS88-6 – 80KA at 400V (50,000 amps)

16.5KA at 230V (16,500 amps) What size do you think the one shown above is?

This one

Miniature circuit breakers (MCB’S) (BS EN 60898) Formerly BS3871 The over-current protection of these devices is achieved by the choice and setting of the bi-metallic strip that is contained within the body of the device.

The bodies of the devices are not colour coded and vary in shape and size between manufacturers. It is apparent though from observing the body of the device as to its rating and trip characteristic i.e. its type.

These devices are classified into main categories for short circuit rating. The ratings of these are always apparent by observing the body of the device. Always look for the large rating inside a rectangle.

BS3871 (Replaced by BS EN 60898) – What rating and type do you

think the one shown above is?

What is the short circuit capacity of the MCB on the left?

Its type refers to how the device will handle overload.

Type B operates instantly at certain levels of overload where as a type D will handle

that level of overload for a slightly longer spell of time.

This is particularly useful for inductive load circuits.

Residual Current Circuit Breakers BSEN61009 and Residual Current Devices BSEN61008

Ratings of these devices are as per BS EN 60898 devices in both over-current and short circuit protection. They can be identified in exactly the same way. So, how do we know whether a device is a residual current device?

The thing that makes them unique is the fact that they bear the rating of their earth leakage protection rating too. This is shown in either milliamps (mA) or amps (A) usually shown like this:

(I∆n 30 mA). The mA rating indicates the amount of earth leakage current that it takes to operate the device.

See the pictures below.

Now answer the questions below Can you see the residual

current rating of the device above? Write it down.

Can you see the residual current rating of the device above? Write it down.

1. What does the overload rating on a protective device indicate?

2. What does the short circuit rating on a device indicate?