Basic Electrical Q's!!

[stevehayes]

Hi

I have some baaic electrical questions that I need some help on please if possible?!

What is the difference between a MCB and a RCD? I know a MCB is like a fuse and trips a circuit if the current gets overloaded, but what exactly s and does a RCD do?!!! Sorry if I sound ignorant!!

Second point-how do you work out or calculate the thickness of cable needed to run a certain appliance. For eg if I need to replace my Immersion Heater, is a 2.5mm 2 cable a necessity ? How do you work out how thick a cable should be so you know what type to be!!?

Thanks for your help and advice!

 

[holmslaw]

..

 

[ban-all-sheds]

What is the difference between a MCB and a RCD? I know a MCB is like a fuse and trips a circuit if the current gets overloaded, but what exactly s and does a RCD do?!!! Sorry if I sound ignorant!!

http://en.wikipedia.org/wiki/Residual-current_device

http://www.westernautomation.com/pages/demystify.htm

http://www.memonline.com/guide06.html

Second point-how do you work out or calculate the thickness of cable needed to run a certain appliance. For eg if I need to replace my Immersion Heater, is a 2.5mm 2 cable a necessity ? How do you work out how thick a cable should be so you know what type to be!!?

http://www.kevinboone.com/cableselection_web.pdf

 

[Adam_151]

Hi

What is the difference between a MCB and a RCD? I know a MCB is like a fuse and trips a circuit if the current gets overloaded, but what exactly s and does a RCD do?!!! Sorry if I sound ignorant!!

An RCD protects against faults to earth, basically, it knows that the currents in the line and neutral conductors should be of equal manitude, and if they are not, then the residual most be returning via an alternate path (either through poor insulation, or a poor fellow being shocked, etc) and cuts off the power

Second point-how do you work out or calculate the thickness of cable needed to run a certain appliance. For eg if I need to replace my Immersion Heater, is a 2.5mm 2 cable a necessity ? How do you work out how thick a cable should be so you know what type to be!!?

You work out the load that the cable must carry (Ib), choose an appropiate protective device (In), then you decide how the cable will be installed and pick an appopiate installation method, you then identify any further factors that will apply, and apply them to In to find a minimum tabulated value (Iz), you then consult the correct table in BS7671 for your cable type, and read down the column for the correct install method until you find a cable that has a rated current carrying capacity that either matches are exceeds this value, you then go back and check the cable meets requirements for volatage drop, earth fault loop impedance, and thermal withstand of faults

... well you did ask

2.5mm² for an immersion heater is very likely to be correct, unless there is something exceptional going off, sometimes 1.5mm² might be sufficent, but thats corner cutting!

Thanks for your help and advice![/quote]

 

[ban-all-sheds]

An RCD protects against faults to earth, basically, it knows that the currents in the line and neutral conductors should be of equal manitude, and if they are not, then the residual most be returning via an alternate path

I didn't know you were American....