All final circuits must have a CPC
All CPCs must be connected to the star point of the local transformer with a low resistance connection.
Max Ze
Each system has a maximum impedance Ze Allowed under ESQCR 2002
TNS 0.8Ω
TNC-S 0.35Ω
TT 21Ω at the transformer 200Ω at ccu
When the supply cable is SWA, it is normally 2-core (L&N) and the steel armouring is used as the earth conductor.
In a TNCs system the earth and neutral are seperate in the final circuits as usual. But they are combined in the supply from the transformer.
Maximum disconnection times for 230v AC supplies using TN earthing systems
Final circuits up to and including 32A 0.4s
All other circuits (incl. distribution circuits) 5s
Disconnection times
The more current that flows the faster the disconnection time.
To achieve fast disconnection times we need lots of fault current.
We must design our circuits so that if a fault occurs, then as much fault current will flow as possible.
To ensure that fault current are high, ohms law tell us we must have a low impedance - for the whole of the fault loop Zs
short circuit
A live conductor is any conductor that carried current in normal service (no faults)
For single-phase circuits a short circuit could be contact between line and neutral L-N
For three phase circuits a short circuit could be contact between line and neutral L-N or between phases L-L (400v)
Short circuits can have very little resistance between conductors so the fault current can be as high as 1,00s or 10,000 of amps, sometimes more.
Extra hazards
What are the extra hazards and risks with electricity on-site?
mechanical damage from: construction activities
Vehicles
Plant
Abrasive materials
Weather - IP codes.
Chemicals - corrosive - acid/alkali.
Earthed metal - higher fault current
Voltage bands.
CTE (Centre tapped earth) Transformer
Step down tranformer.
230v:110v
Protective multiple earthing (pme)
What is it and when is it required?
PEN (protective earth and neutral)
Hazrds of overhead conductors
Contact with line conductor.
What happens if there is a break in the pen?
No current flows so there is no volt drop
any circuit that is switched on at thr time connects the neutral to the line conductor through the load. so the neutral becomes live.
All the neutrals are connected in the ccu so all the neutrals become live .
Also because it is TNC-S the neutral tail is connected to the earthing system in the cutout .
Anything that is earthed or bonded becomes life. I.e all exposed and extraneous conductive parts have 230v
also because no current can flow, all mcb's and rcb's stop working.
Solution: PME (protective multiple earthing)
Extra earth electrodes are connected to the pen conductor at regular intervals along the supply
This forms a mini TT system around the break in the pen conductor, connecting neutral and earth.
Keypoints:
PME consists of earth electrodes connected to the PEN conductor at regular intervals along the supply.
It provides an alternative route for neutral and fault (earth) currents if the PEN conductor is damaged
PME is required for two-wire overhead supplies to TNC-S systems
Before converting TT to TNC-S, it must be confirmed by the supply company that the supply is PME.
Overcurrent devices 1
Device characterstics
overcurrent has 3
-Rating
The lowest current that can make the device opperate.
MCB's usually avaliable as 6A, 10A, 20A, 32A and 40A or 45A.
-Breaking capacity
The highest current that the device can safely interrupt
Very large currents passing through fuses or MCB's can create extremely dangerous situations
Fuses:
Overcurrents normally cause the fuse element to melt
Very large currents cause the fuse element to vaporise with such force it can rupture the fuse casing resulting in explosion and fire.
This can damage or obliterate nearby equipment.
MCB's:
As well as thr possibility of components exploding, MCB's can present another hazard under certain circumstances.
Contact breakers are two solid conductors held together inside each MCB.
When the MCB operates, these contact breakers are allowed to spring apart.
So every overcurrent device can cause serious problems if too much current passes through it. But how much is too much?
Any current above the breaking capacity of the device means that the device may not safely interrupt the current, (ie not safely disconnect the circuit)
-Type (B,C or D)
Some appliances and fixed loads draw more current when they are switched on, then settle to their full load current.
They are known as: INRUSH CURRENTS
Inrush currents are usually caused by inductors.
Type B
Instantaneous trip above 5x rating, All circuits that have only resistive loads
Type C
Instantaneous trip above 10x rating
Examples: Moderate inductive loads:
Small or meduim motors
transformers
flourescent lighting circuits.
Tybe D
Instantaneous trip above 20x rating.
Examples of heavy inductive loads:
Large motors
Electricl arc welding equipment
X-ray machines
Small particle accelerators.
Fuses that allow inrush currents are also avaliable.
Protective device Bs No KA
semi enclosed/rewirable BS3036 1-4
Cartridge Fuse
Plug top type BS1362 16.5
Cut out type BS88-3 33
HBC/HRC
High breaking capacity BS88-2 Up to 80
High rating capacity
MCB's BS60898 usually 6
RCD's BS61008 Varies
RCBO's BS 61009 same as MCBs
Summary: you need to know:
the 3 charactistics of overcurrent devices
Standard ratings of mcb's
Efects of current
Any current will produce a magnetic field
more current produces a stronger field
constant DC current produces a constant field.
AC current produces a constantly changing alternating field
Sometimes this is very useful: motors, transformers, solenoids, relays etc.
Sometimes it is a nuisance: stray inductance, radio interference etc.
Semi enclosed fuse, BS 3036
These fuses are an older style of fuse, but there are many instsllations still using them.
If the fuse blows the element (wire) is replaced by the consumer!
the fuses are colour coded
White 5a
blue 15a
yellow 20a
red 30a
green 45a
Cartridge fuse BS1362 plug top
Max rating 13a
Also used on FCU's for fixed loads.
Cartride fuse BS88-3 Usually rated at 100a for domestic supplies but can be less, 80a, 60a or sometimes 40a in very old properties.
BBC or HRC fuses to bs88-2
HBC = high breaking capacity
HRC = high rupture capacity
They are very solidly built to withstand high fault currents and the explosion of the fuse element that results. Hence breaking capacities as high as 80KA.
Fuses and faults
The principle of operation for all types of fuse is: thermal (heat)
When a normal amount of current flows.
IF the current increases and exceeds the rating, the fuse element melts and breaks the circuit.
Arcing
If current is flowing through a circuit and a break occurs, an arc can form just as the break is starting to open.
The arc can then be stretched out as the gap widens
Arcing can produce heat damage to switches, isolators and protective devices.
Also it means current can still flow through devices that
MCB's miniture circuit breaker to BS 60898
Identify the various parts of the MCB
Residual current device
Leftover current device
Discriminations between ciruits
This is achieved by having seperate overcurrent protection, each circuit has its own fuse/mcb
Eath fault protection also needs discrimination. This is often achieved by having circuits grouped together and protected in groups by more than one rcd.
Having progressively lower rated cpds as we go from the supply towards the load, means that the lowest rated device upstream from the fault will operate, thereby making the circuit safe while all other devices and all other circuits are unaffected.