navigation of bs7671 Flashcards
index
525
part 1
110.1 gives a list of items it covers 110.2 what it doesn’t cover
basic protection and fault protection
131.2.1. 132.2.2
thermal effects
131.3
132.5.1 environmental factors
ip ratings to see what requirements are needed.
part 4
most often referred to. about electric shocks , over current and thermal effects
410.3 states that we can use one of the following methods to the risk of electric shock
- automatic disconnection of supply .
- separation by SELV.
3.
protective earthing
3 points to consider
exposed conductive part sare connected to a protective conductor ( cpc )
- conductors willl comply w chapter 54
- a cpc shall be run and terminated at each point in the wiring and at each accessory. whether it’s class1 or class2.
411.3.2.2 and table 41.1
gives max disconnection times for final circuits. if don’t get Max times on 41.1 we add an RCD or look at reg419.
411.3.2.3
allows 5 s disconnection time for any circuit that is a distribution circuit.
additional requirements for circuits with luminaries
requires additional protection by a 30ma RCD
core requirements for TN systems
- 4.1 - protective earthing
- 4.3 - functions of N and cpc ( PEN )can be combined in a single conductor provided no isolating device is installed
devices that may be used for fault protection for tn systems.
- an over current protective device. e.g a fuse or circuit breaker
- RCD - ( can’t use in tncs) operates by looking for differences in live conductors. must be backed up by a CB for L-L OR L-N faults.
selecting the device to achieve disconnection times
411.5.4. cmin ?
Uo divided by Zs X cmin - current - look @ tables
what is the difference between circuit breakers
type b circuit breakers are the standard ones
type c deals with inductive loads e.g fluorescent lighting
type d cBs are for highly inductive loads. e.g motors.
value in regs books for 41.2 3 nd 4 is tabulated.
measured value is 80 % !!!!!!!!!!
TT system
use a earth spike so fault path is down the spike into the ground back to the starhoint, completing the loop in that way
basic requirements of a TT system
- exposed and extraneous conductive parts are still connected to the M.E.T
- The MET is connected to the earth electrode
- the neutral is typically earthed
for ADS. RCD & OCPD may be used
if we use ocpd for ads for a tt what equation do we use
uo divided by zs X cmin = current
IT systems
411.6
exposed conductive parts are to be earthed
in ac systems the earthing it to limit touch voltage to 50v
V=IxR
IT systems are systems in which all live parts are connected to earth ( or connected to earth through a high impedance )
- during the first fault to earth , the system typically remains safe
- however if a 2nd fault occurs , there is a high chance of electric shock is apparent
protective devices for it systems
- insulating monitoring devices (IMDs)
- residual current monitoring devices (RCMs)
- insulation fault location systems (IFLS)
- overcurrent protective device ( OCPD)
- residual current devices ( RCD’s)
Functional extra low voltage
usually no electrical separation between primary and secondary . voltage comes down our primary , along our windings , & down the neutral
Reduced low voltage ( RLV) system
where we want to reduce the voltage to earth to help maintain safety and where its not always to increase conductor sizes or earth electrode resistances.
what is reinforced insulation
- 2 layers of insulation ( both basic and supplementary)
- or 1 layer of insulation that is reinforced.
electrical seperation
primary and secondary are separated electrically so under fault conditions you wont get an electric shock.
requirements :
1. isolating transformer. 2. 500 v maximum
3. no earth connection on the secondary 4. only for individual items of equipment 5. multiple items of equipment each need their own transformer.
electrical seperation
e.g shaver points.
primary and secondary are separated electrically so under fault conditions you wont get an electric shock.
requirements :
1. isolating transformer. 2. 500 v maximum
3. no earth connection on the secondary 4. only for individual items of equipment 5. multiple items of equipment each need their own transformer.
SELV AND PELV
separated and protective
selv and pelv provide extra low voltage through safety isolating transformers. both are extra low voltage systems supplied from safety isolating transformers.
selv
in selv systems both conductors are isolated from earth and conductors of other wiring systems
- when the voltage is low enough it is often used in places where risk may be higher e.g bathrooms
pelv
in pelvic systems 1 conductor is earthed but otherwise same as selv & is used in systems that require an earth connection for electrical interference control ( EMC ) or to detect broken conductors.
use of rcds for additional protection
- when used they should not exceed 30ma
- the use of a rcd as a sole means of protection is not appropriate and must be used with one of the following measures :
- ## pg 73
supplementary equipotential bonding
is designed to reinforce the faradays cage principle within an installation. usually used where if the 50 V dc touch voltage were exceeded greater danger may arise than in a normal installation. e.g bathrooms
- may also be used to limit the touch voltage if disconnection times are not achievable in 419.3
supplementary equipotential bonding
is designed to reinforce the faradays cage principle within an installation. usually used where if the 50 V ac touch voltage were exceeded greater danger may arise than in a normal installation. e.g bathrooms ,
- may also be used to limit the touch voltage if disconnection times are not achievable in 419.3
basic insulation of live parts
live and neutral conductors must be completely covered by insulation and must only be removable by destroying it.
barriers and enclosures
- 2.1 states that live parts are protected to prevent access from objects in accordance to IPXXD OR IP2X which is finger proof - 12.5 ml by 80 ml
- regulation 416.2.2 requires horizontal top surface to be at least IP4X - this is 1mm
416.2.4 where it is possible to gain access to a enclosure containing live parts, this shall only be possible if :
- by the use of a key or tool
- where the barrier has a protection of at least ip2x to prevent live parts contacting the key or tool.
protective measures for application where installation is under supervision of a competent person
- non conducting location
- protection by earth free equipotential bonding
electrical seperation
placing out of reach
1.25m sideways 2.5m Up
electrical separation of supply to one or more current usingg equipment
exposed conductive parts are connected by protective bonding conductors. these bonding conductors are not earthed.
- there are limits on the length of wiring
- additional precautions are there to protect the wiring system from damage and insulation failure.
- when ADS is not feasible.
aimed at limiting touch voltage to 50 V
- 2 - permits reducing voltage to a certain level
- 3 - local eqi bonding to lower the touch voltage.
arc fault detection devices
- recommended to provided extra protection against fire caused by arc faults in final circuits.
- AFDDs are to be placed at the origin of circuits to be protected.
consumer units in domestic properties need to
- be of a non combustible material
- be enclosed in a enclosure or cabinet of non combustible material
what is a AFDD
- it is a circuit breaker that operates ( opens) when an arc fault is detected. - outwardly they look similar to an RCD and have a test button to check operation.
- may also be part of a combined device , along with an CB , RCD or ever RCBO.
Cables for evacuation in an emergency
- cables must meet BS for fire conditions
- not be installed in arms reach
- cable system of non flame propagating type
Luminares must be :
- suitable for the location
- have the appropriate IP rating
- limited surface temperature
- prevent build up of dust
combustible materials.
- fire propagating structures. e.g grenfell.
what is chapter 43 about
- overload & overcurrent
- fault current - referred to as fault of negligible impedance
ocpd will disconnect all Line conductors.
protection of line and neutral conductor
- L & N conductors are both to be protected from overcurrent
nature of protective device
- each device must be able to make and break the current.
protection against overload
- links to cable calculations. 1. ib 2. it etc
- if reewireble we bring a factor
- buried cables bring another factor
radial vs ring
- can’t cover as much floor area w radial
- ## ring circuit t uses a lot less copper so cheaper
part 5 of bs7671 mutual detrimental influence
- selecting and installing equipment in such a way to avoid harmful effects influenced by different systems being installed in close proximity to each other.
types of RCD.
type A - used for general purposes
type Ac - can’t detect D.C. e.g vehicle charging. if theres an earth leak it will stop the rcd working.1
type F -
type B -
