Joined: 11 Jan 2004 Posts: 16759 Location: Cheshire, United Kingdom Thanked: 24 times
Posted: Tue Jun 29, 2004 3:38 pm Post Subject:
2) CHANGING A CONSUMER UNIT
If you need to add a consumer unit, or change the existing one, the best course of action would be to call the Regional Electricity Company (REC), and ask them to fit (or install one yourself and ask them to wire in) an isolator.
This is fitted in between the tails from the meter and the CU itself which means that the tails to your CU are also isolated thus making the CU absolutely safe to work on once the isolator is open.
If you want to, you can always fit a Henley block in between the new isolator switch and the CU, allowing you to fit extra CU's in the future.
PLEASE DON'T tamper with the cut-out seal or cut out or tamper with meter seal or tails.
This will render you liable to prosecution, and may even be lethal. One slip is all it takes - I'm not trying to scare anyone, but I've seen it done, and it's not pretty.
Leave the meter seals, cut out etc to the REC, and once you (or they) have installed your isolator you can work on the system in complete safety without bothering the REC again.
======A============B=========C=======D=======E
=: cable
A: cutout
B: meter
C: isolator
D: Optional Henley Block
E: CU or CU's.
Last edited by securespark on Sat Mar 01, 2008 2:50 pm, edited 1 time in total
Joined: 28 Mar 2004 Posts: 7019 Location: United Kingdom Thanked: 25 times
Posted: Fri Aug 27, 2004 10:15 pm Post Subject:
5) ADDING A THIRD SWITCH TO A 2 WAY SYSTEM.
You don't have to understand the workings of the existing 2 way system, you just have to remember that the combination of a 2 way switch and an intermediate switch act like a 2 way switch that can be controlled from 2 places.
Follow this diagram
Click image to enlarge
Last edited by plugwash on Sat Feb 11, 2006 8:02 pm, edited 2 times in total
I would strongly recommend a copy of the Wiring Regulations, and also (but not as strongly) copies of the Guidance Notes, but bear in mind there were new versions out in 2008 onwards, as the Wiring Regulations were revised. A lot of people on eBay are still selling old versions, pay close attention, and don't buy the 16th Edition ones by mistake.
The Electrician's Guide shown above is a good explanation of the regs, but if you fancy one with a bit more of the electrical engineering theory behind it all, I can recommend this:
It is a commentary on the 16th, not the 17th, and the new one will be out soon, but if you can find a cheap second hand copy it will still be useful and informative, as the underlying theory never changes.
Last edited by ban-all-sheds on Sat Oct 31, 2009 9:50 am, edited 21 times in total
Joined: 27 Aug 2003 Posts: 23507 Location: London, United Kingdom Thanked: 187 times
Posted: Tue Sep 21, 2004 10:32 am Post Subject:
7) MCBs
Reason says that there ought to be a Type A MCB, but I've never seen any info on one.
B, C, and D have different time/current curves for tripping. Type B will trip faster than type C for a given over current, and type C will be faster than type D. The other way of looking at it is that type Ds need more current than type Cs, which need more than type Bs, to trip in a given amount of time.
In summary:
Code:
Type Will not trip in Will trip in
100ms at rating 100ms at rating
B 3 x 5 x
C 5 x 10 x
D 10 x 20 x
This chart shows different curves superimposed:
Click image to enlarge
Why does this matter?
One of the jobs of a protective device is to disconnect the supply in the event of an earth fault - i.e. if something goes wrong with your fan heater or toaster and a live conductor makes contact with the earthed case, then a current will flow to earth. What we need is for that current to get large enough for the fuse to blow or the MCB to trip.
The situation of current happily flowing to earth without tripping the breaker, and therefore with the case of the toaster remaining live is a Bad Thing.
We also want the current to get large enough quickly so that the case doesn't remain live for very long, and the earth conductor doesn't have time to get hot enough to melt. This is called the disconnection time, and clearly, as I=V/R, we need a low resistance to get a high current. A C type breaker needs a higher I to trip quickly than a type B does, so a circuit protected by a type C needs a lower earth loop resistance than one protected by a type B.
How much lower depends on how fast you want it to disconnect. The wiring regs say that a socket circuit should have a disconnect time of 400ms.
Lighting circuits, by contrast, require a 5s disconnect time,and there is a case to be made for using type C breakers on lighting circuits because they are much less prone to tripping with the brief current surge that occurs when a lamp fails than a type B is.
At 5 seconds the curves for type B and C breakers have almost met, e.g. there is very little difference in the earth loop resistance limits for a circuit with that disconnection time.
Not so at 400ms though. The typical maximum values, in ohms, for the earth loop resistance (Zs), for 32A type B and C MCBs are:
Code:
Type 0.4s 5s
B 1.50 1.71
C 0.75 1.6
As you have a TN-C-S supply, you might get away with type Cs on socket circuits, as the maximum allowable external component of the earth loop impedance (Ze) for that type of supply is 0.35 ohms. (And of course the DNOs strive ceaselessly to ensure that this limit is never breached. )
For a TN-S supply, Ze is allowed to be 0.8 ohms, so type C breakers are flat-out not allowed for socket circuits, but even with TN-C-S, you can see that there's not much left for your R1+R2 (the combined resistance of the live and earth wires in a circuit).
With a TT supply, Ze could be very high, which is why RCD protection is mandated in the regulations.
That, briefly is why you'll see people expressing concerns about earth-fault-loop impedance's.
Last edited by ban-all-sheds on Thu Jun 29, 2006 8:22 am, edited 4 times in total
Joined: 27 Aug 2003 Posts: 23507 Location: London, United Kingdom Thanked: 187 times
Posted: Wed Oct 06, 2004 8:22 am Post Subject:
8 ) SIZING MCBs & CABLES
Note - this considers overload protection only. Other factors such as voltage drop and disconnection times need to be considered when designing a circuit. Also, it only quotes the example tripping/non-tripping currents for Type B MCBs, not cartridge or rewirable fuses, which are not the same.
More detail on these aspects, and factors which affect a cable's capacity can be found here.
Protective devices have 3 values of interest here:
In: its nominal rating (e.g. 32A)
I1: its non-fusing/non-tripping current. For a Type B MCB this is 1.13In I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour
So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is Ib, then this should not be greater than the rating of the protective device:
Ib <= In
If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:
In <= Iz
Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:
1.45Iz >= I2
So Ib <= In <= Iz
and
I2 <= 1.45Iz.
Or if you prefer pictures:
Click image to enlarge
Running an MCB at or near its limit for long periods will cause it to overheat, and eventually weaken. But you shouldn't do that - look at the diagram, Ib is lower than all the rest.
Last edited by ban-all-sheds on Thu Jul 03, 2008 9:02 am, edited 2 times in total
Joined: 27 Aug 2003 Posts: 23507 Location: London, United Kingdom Thanked: 187 times
Posted: Wed Oct 13, 2004 1:33 pm Post Subject:
9) HALOGEN LAMP BASES
There are 2 common types of mains halogen lamp bases,GZ10 and GU10.
GZ10 lamps are the dichroic (cool beam) type, where a lot of heat comes out of the back, and GU10s have internal reflectors (the lamps are also known as PARxx - Parabolic Aluminium Reflector<size>) where the heat comes out forwards.
If you look you'll see that you can put a GU10 lamp in a GZ10 fitting, but not the other way around. This was done so that you couldn't put a dichroic lamp into a luminaire designed for PAR lamps, as the rearward-reflected heat would damage it.
Well guess what - people have now started making dichroic GU10s
If a luminaire has a GU10 base, then unless the instructions explicitly say otherwise, you must only use PAR GU10s in it, not dichroic.
Last edited by ban-all-sheds on Tue Sep 04, 2007 11:04 pm, edited 1 time in total
Joined: 14 Sep 2004 Posts: 3972 Location: United Kingdom Thanked: 0 times
Posted: Sat Dec 18, 2004 5:25 pm Post Subject:
12) CABLE INFO
Since people keep saying 'there is a 3 core' when it is twin and earth etc, this is a guide to what cable it is:
Twin and Earth (T+E)
This has 2 core carrying the load and an earth. This is NOT 3 core!
3 Core and Earth (3C+E) (mostly known as 3 core)
This has 3 cores for carrying the load and an earth. Mostly used in 2 way lighting circuits and interlinking smoke alarms. Also used to power a fan, i.e permanent live, switched live and neutral.
Last edited by andy on Sat Dec 18, 2004 6:15 pm, edited 3 times in total
Joined: 27 Aug 2003 Posts: 23507 Location: London, United Kingdom Thanked: 187 times
Posted: Fri Dec 31, 2004 1:14 pm Post Subject:
13) ABBREVIATIONS
CCT - Circuit
CCU - Cooker Control Unit
CPC - Circuit Protective Conductor
CU - Consumer Unit
EEBAD - Earthed Equipotential Bonding And Automatic Disconnection Of Supply
ELV - Extra Low Voltage = Below 50V AC \ 120V Ripple Free DC
FCU - Fused Connection Unit
FELV - Functional Extra Low Voltage
HBC - High Breaking Capacity
HRC - High Rupturing Capacity
HV - High Voltage
LV - Low Voltage = 50V - 1000V AC \ 1500V Ripple Free DC
MCB - Miniature Circuit Breaker
MCCB - Moulded Case Circuit Breaker
MD - Maximum Demand
MICC - Mineral Insulated Copper Cable aka Pyro
PAT - Portable Appliance Testing
PELV - Protected Extra Low Voltage
PEN - Protective Earthed Neutral
PFC - Prospective Fault Current
PME - Protective Multiple Earthing
PSCC - Prospective Short Circuit Current
PVC - Poly Vinyl Chloride
RCBOs - Residual Current Breaker With Integral Overload Protection
RCCB - Residual Current Circuit Breaker
RCD - Residual Current Device
SELV - Separated Extra Low Voltage
SRCBOs - Socket Outlet Incorporating RCBO's
SWA - Steel Wire Armour (Cable)
UPS - Uninterruptible Power Supply
VD - Voltage Drop
--------------------------------------------------------------------------------
A - Amp
W - Watt
V - Volt
R - Resistance
Z - Impedance
mA - milliampere
kW - Kilowatt
kV - Kilovolt
Joined: 27 Aug 2003 Posts: 23507 Location: London, United Kingdom Thanked: 187 times
Posted: Tue Feb 01, 2005 2:27 pm Post Subject:
14) BORROWED NEUTRALS and why they are a BAD THING
A borrowed neutral is where something (nearly always a light) has its live connection on one circuit and its neutral on another:
Click image to enlarge
This is most commonly encountered with 2-way switching of a landing light. The live originates on the downstairs lighting circuit and goes through the downstairs and upstairs switches to the landing light, where of course a neutral is also needed, and the easiest place to get one is from the upstairs lighting circuit.
It was done like this for years, with never a problem because people only had 1 lighting circuit. The problem arises when the lighting is split into two - Upstairs & Downstairs.
The reason it's a problem is shown below. Consider the two circuits as before, and you want to break into Circuit 2 at point X to install a new light. So you switch off the MCB, or pull the fuse, verify that the circuit is dead, and cut the cable.
At that point some or all of the neutral cable in Circuit 2 becomes live via the path through the light from Circuit 1.
Last edited by ban-all-sheds on Thu May 15, 2008 7:10 pm, edited 2 times in total
Joined: 18 Jul 2004 Posts: 5377 Location: Devon, United Kingdom Thanked: 33 times
Posted: Sat Mar 19, 2005 11:33 am Post Subject:
15) EARTHING ARRANGEMENTS
There are 3 main commonly used Earthing Arrangements found in domestic installations.
TN-C-S ; TN-S ; TT
The T stands for 'TERRA' which is French for Earth.
The C stands for 'COMBINED' in relation to the neutral and the earth.
The S stands for 'SEPARATE' in relation to the neutral and the earth.
TN-C-S
A TN-C-S system has the neutral and earth COMBINED in the DNO's supply cable(s), and SEPARATE
within the installation, (this being the meter position and onwards).
Click image to enlarge TN-S
A TN-S system has the neutral and earth SEPARATE throughout the entire system, including the DNO's Supply cable(s).
Click image to enlarge
OR
Click image to enlarge
OR
Click image to enlarge TT
A TT system has the neutral running in the supply cable(s) as normal, and the earth from the installation
is connected to an appropriate Earth Electrode inserted into the ground.
An RCD is essential in an installation formed by a TT earthing arrangement.
Click image to enlarge
It is the DNO's responsibility to provide a TN-C-S or a TN-S supply, if requested and available,
it is not their responsibility to provide a TT earthing arrangement.
In order for the DNO's to provide a TN-C-S supply, they have to construct their network to the
standards of PME (protective multiple earthing), this is why a TN-C-S system is often referred to as a PME installation.
Last edited by Lectrician on Sun May 28, 2006 11:42 am, edited 7 times in total
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