Protective Devices For The AM2 Exam Cicuits

During the new AM2 exam, as a part of the composite installation you will be required to determine and select the appropriate protective device for every circuit.
You will be provided with a selection of protective devices but among the right ones there will also be some which are not suitable for the given circuit. You will be expected to know which ones to avoid for a given type of circuit.

At the end of this text you will find the correct selection of protective devices for every circuit in the AM2 composite installation exam. It is vital for you to know how to choose the right protective device for a certain circuit in order to be competent for electrical installation work. We have prepared a short guide just for this.

Determining the protective device characteristics for circuits in the AM2 exam

Electrical installation design can be a long process. However, in the AM2 exam there are given details for every circuit, which makes the design much easier and faster. All you need to know is to use the given details to select the appropriate protective device for a given circuit.

The main formula we can start with:
Iz >= In >= Ib
Ib - design current of circuit (A) (the load to be connected to the circuit)
In - nominal current of protective device (A)
Iz - effective current carrying capacity (A)

This means that in every circuit at all times the current carrying capacity of the cables must be higher than the highest possible load current. Also the protective device rating must be higher than the load current but lower than the maximum current carrying capacity of the cable installed.

Now let's go through the AM2 exam composite installation circuits:

Circuit 1: CO detector in FB200 type cable (AM2 Exam)

Given are two very important details. One is that the CO detector uses very little energy, even less than 1A. The other given detail is the size of the FB200 cable provided which is 1.5mm2.
Starting from Table 4D1A in Appendix 4 of BS7671 IEE Wiring Regulations we can determine the current carrying capacity for any given cable.

It is recommended to remember the values for the most common cables in use like the 1.5mm2 and 2.5mm2 twin and earth or the 2.5mm2 singles cable.

The 1.5mm2 cable can handle up to 16A depending on other factors which are not relevant right now. This means that our protective device could be selected up to the size of 16A. Considering that our CO detector uses less than 1A current, we can see that the 16A rated protective device is too big for this load. We can choose the one which is closest to our demand and readily available; that would most certainly be the 6A MCB protective device.
To determine the type of the protective device you will have to know which type is suited for which situation. There are many old and new type MCB`s on the market today but the most common ones are the C, B, and D types. The letters indicate the instantaneous tripping current of the device:

Table 1. Short guide to the types of MCB`s
Type B	Above 3 In up to and including 5 In	Suitable for domestic use
Type C	Above 5 In up to and including 10 In	Suitable for commercial and light industrial use
Type D	Above 10 In up to and including 20 In	Suitable for industrial use

After assessing all the details we can determine that the CO detector circuit installed in FB200 1.5mm2 cable is best protected with a 6A Type B MCB.

Circuit 2: The lighting circuit (AM2 Exam)

To determine the size and type of protective device for the circuit once again we look at the given details:

• There is only one 28W fluorescent lamp to be supplied with a current consumption of less than 1A
• 1.5 mm2 twin and earth cable is provided for the circuit with a maximum current carrying capacity of 16A

Following the same steps as with the first circuit, we can determine that the ideal protective device for this circuit will be the 6A Type B MCB.

Circuit 3: BS EN 60309 16A socket outlet (AM2 Exam)

Given details:

• Maximum load will be not more than 16A
• 2.5mm2 SWA cable with current carrying capacity of more than 20A is provided for the circuit
• Since it is unlikely that a 16A socket outlet would be installed in a domestic environment, a Type C MCB is recommended.

From these details we can determine that the ideal protective device for this circuit will be the 20A Type C MCB.

Circuit 4: 13A socket outlet ring circuit (AM2 Exam)

Given details:

• Ring circuit with 13A socket outlets
• 2.5mm2 twin and earth cable

The Ring Circuit or Ring Main is a unique circuit arrangement which is preferred in the UK. It is usually used in domestic environment, with 2.5mm2 cable connected in to the ring formation and protected by a 32A Type B MCB.  The size of the protective device is suitable because the 2.5mm2 cable is connected in ring formation which will effectively double the current carrying capacity to each socket on the ring. One un-fused spur is allowed from every socket to only one double or single socket in 2.5mm2 cable.
Because of the nature and use of the ring circuit in the AM2 exam we do not need to take into consideration the area covered or the circuit length regarding voltage drop for the circuit.

Given the details we can determine that the ideal protective device for this circuit is the 32A Type B MCB.

Circuit 5: S Plan central heating and hot water system with a solar thermal sustainable energy element (AM2 Exam)

Given details:

• 2.5mm2 heat resistant flexible cable with current carrying capacity of more than 20A
• 2.4mm2 PVC singles cable with current carrying capacity of more than 20A

Because this system forms a part of the AM2 exam, some parts of the installation are not physically present but are simulated instead, so we cannot know for sure what the current load will be.
According to design rules ( Iz>=In ) our protective device must be of that size so that it can safely protect the cables in the installation.

Given these details we can determine that the ideal protective device for this circuit will be a 16A Type B MCB or a 20A Type B MCB.

Circuit 6: 3-phase direct on line motor/starter circuit (AM2 Exam)

Given details:

• 1 KW motor with a current load of about 5A
• 1.5 mm2 SY cable with maximum current carrying capacity of 16A
• 1.5 mm2 PVC singles cable with maximum current carrying capacity of 16A

Even if the motor is not physically there we still need to bear in mind that it can be connected at any time. A motor on a DOL starter (Direct On Line) will have a much higher starting current (up to six times) than the one under normal operation. However, this starting current will have an effect only for the first milliseconds of the start, which can be dealt with a higher rated C type MCB.

Given the details we can determine that the ideal protective device for this circuit is a 10A Type C MCB.

This table states the correct selection of protective devices for every circuit in the AM2 composite installation exam:

Circuit	Protective device
CO detector circuit	6 A Type B MCB
Lighting circuit	6 A Type B MCB
16 A socket outlet circuit	20 A Type C MCB
13 A socket outlets ring circuit	32 A Type B MCB<
S plan heating system with solar thermal  circuit	16 A Type B MCB
DOL motor starter circuit	10 A Type C MCB