Luke1 said:
Right, well im a training spark and i had a few questions i wanted clearing up.
1, Why does a double insulated load not require an earth, whats so different about it to any other load, i mean why cant you get a shock off it?
This is generally Class II equipment in which protection does not rely upon basic insualtion, but this is augmentary by secondary insulation. Further there is connection for exposed metal parts to any protective condictor and nor should any be provided, they must not rely upon the fixed woring of an installtion for this protection either. All such equipment must comply with BS2754:1976
Luke1 said:
2, There are different types of MCB's arnt there? like A,B,C and a few others. Can someone please explain in what senario who i use eash.
Apart from compliance with BS7671 the importance of the "types" is to ensure that the MCB does not cause nuisance tripping when equipment on that circuit is energised or started. This can cause damage to the equipment, the MCB and also prove dangerous to any person using the equipment.
For instance, electric motors or low voltage lighting the use of a type B MCB may cause nuisance tripping and the choice of a type C MCB will invariably solve this problem.
The third characteristic is the short circuit protection. This is intended to protect against heavy faults, that may be rated in the thousands of amps, caused by short circuit faults. The capability of the MCB to operate under these extreme conditions gives its short circuit rating in Kiloamps (kA). In most domestic, small commercial and office environments a rating of 6kA is adequate. However in heavy commercial and certainly Industrial installations ratings of 10kA or more may be required.
MCBs are offered in types B , C and D. Where normal resistive loads are being fed the type B MCB may be sufficient however in many heavy commercial or industrial applications heavy loads, such as motors and special lighting exist, which may adversely affect the MCBs during starting and it may be necessary to use type C breakers.
In some cases such even these MCBs may not be sufficient. In some industrial or even commercial installations the start-up characteristics of the circuit may be such that even a Type C may not withstand the inrush currents, consequently a Type D breaker will be employed. However the use of Type D breakers whould always be done with care, the operating characteristics of these are such that the circuits may not then comply with BS7671 for normal disconnection times. If there is ever any doubt, expert advice should always be sought to prevent potentially dangerous situations.
Here are the rating of the basic types.
Type B - operates between 3 and 5 times full load current
Type C - operates between 5 and 10 times full load current
Type D - operates between 10 and 20 times full load current
Luke1 said:
3, When bonding a bath room, does a earth have to be taken straight from the mains, but whats happends when there are more than one bathroom? or can it come from the lighting circuit. Also what are the reccomonded size cables used?
There is no requirement for a main equipotential bond to be installed from the distribution board to any room containing a fixed bath or shower. The requirements (601-04) are that all circuits in these rooms should be so bonded to exposed or extraneous metalwork to prevent any potential difference from existing during fault conditions.
Luke1 said:
4, If wiring a garage, do i take say a 10mm cable to it then wire a seperate smaller mains for it, or do i wire it to the mains with its own breakers (for lights, sockets, etc ...) ... if that makes sense
That will depend on any number of factors. The distance from the main supply, the types of circuits being installed and their design currents, the types of equipment that will be used in this installation, and the competence of the people who use said equipment.
Generally I have always found it better to treat the Garage as a seperate installation. Install a time delay RCD (300mA 80A) adjacent to the Main Installation DB, this being fed from the meter not the DB. Then from here I run a suitably sized SWA to a small consumer unit in the Garage. This will normally contain a 63A 100mA RCD main switch , then the circuits for sockets and other such supplies protected by Type B or C 30mA RCBO's. Lighting I would always keep on Type B MCB's for safety reasons.
Always remember, the overload rating of any device installed in the garage should be less than that protecting the cable in the Time delay RCD.
