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17th EDITION REQUIREMENTS FOR THE TESTING OF RCDs :
The 17th Edition of the Wiring Regulations (BS 7671: 2008) will introduce a number of new requirements for the installation of
RCDs, therefore it is timely to look at the requirements within the17th Edition for verification of RCDs. The continuing effectiveness of these RCDs needs to be confirmed periodically. This article discusses the verification required where RCDs are
used to provide automatic disconnection of supply in the event of a fault and additional protection. It should be stated at this point that the 17th Edition does not introduce any significant changes in the requirements for the testing of RCDs even where they are installed to provide automatic disconnection in the event of a fault ,
Use of RCDs to achieve automatic disconnection in case of a fault :
411.3.2.1 requires (in most cases) that a protective device shall interrupt the supply to a line conductor of a circuit or equipment in the event of a fault of negligible impedance between said line conductor and an exposed conductive- part or a protective
conductor for the circuit or equipment within the appropriate required disconnection time. A disconnection time of 5 seconds
for distribution equipment and final circuits of rating exceeding 32A is permitted by 411.3.2.3. Similarly, a disconnection time of 1 second for distribution equipment and final circuits of rating exceeding 32 A is permitted by 411.3.2.4. : 411.3.2.2 states that the maximum disconnection times of Table 41.1 shall be applied to final circuits not exceeding 32 A. Table 41.1 gives the maximum
disconnection times for final circuits not exceeding 32 A of varying nominal voltages forming part of an
installation having either TN or TT system earthing. These disconnection times may be met by the use of fuses, circuit breakers (formerly known as MCBs) or RCDs. is used to meet the requirements of 411.3.2.2, that is, to provide the required disconnection time, the maximum values of earth fault loop impedance in Table 41.5 may be applied. The maximum permissible earth
fault loop impedances (Zs) to ensure RCD operation for non-time delayed RCDs protecting final circuits not
exceeding 32 A are given in Table 41.5, a new table introduced in the 17th Edition, which is reproduced below.
Where an RCD is employed to achieve the disconnection time required by Table 41.1, it is necessary to satisfy
ourselves that the maximum earth fault loop impedance (Zs) stated for a particular sensitivity of RCD in Table 41.5 is not exceeded in the circuit to which they provide protection. This is in effect the same procedure that we applied in earlier editions where fuses
or circuit breakers were used to achieve the necessary disconnection time and indeed continue to apply for fuses and circuit breakers in the 17th Edition. Regardless of which type of protective device is used to achieve the disconnection times required by Table
41.1, whether fuse, circuit breaker or RCD, there is no requirement to confirm that the required disconnection time can be achieved by testing the protective device. Rather, we confirm that the earth fault loop impedance of the protected circuit does not exceed the relevant tabulated maximum earth fault loop impedance for the type / sensitivity of the protective device intended to provide
the required disconnection time.
Maximum earth fault loop impedance (Zs) to ensure RCD operation in accordance with Regulation 411.5.3 for non-delayed RCDs to BS EN 61008-1 and BS EN 61009-1 for final circuits not exceeding 32 A
( 411.3.2.2 ) 230v
TN- Systems : The maximum disconnection time stated in table 41.1 shall be applied to final circuits Not-Exceeding 32Amp
( 411.3.2.3 )
TN- Systems : in a TN-system, a disconnection time Not exceeding 5sec is permitted for a distribution circuit and for a circuit Not covered by Regulation 411.3.2.2 ,
( the table you require TT-systems 41.5 p-50
Resistivity :
Double cable length – Double Conductor Resistance to 1.6 Ω, but halve insulation resistance to 50 MΩ
Halve . CSA – double conductor resistance to 1.6 MΩ, but insulation resistance is unaffected and remains 100 MΩ
Resistivity :
A twin cable has a Phase to Neutral résistance value of 100MΩ and an individual conductor résistance values of ( 0.8Ω )
Determine the values if the cable ….
(a) was double in length ,
(b) length as the same but the conductor cross-section areas was halved ,
Conductor Résistance
This is a function of the resistivity of the conductor material :
In other words , résistance is directly proportional to length and inversely proportional to area ,
So doubling length or halving the area will both double résistance , try it with some values ,
Let’s say the original length is 40m and the area is 2mm2 . The equation then , is :
0.8 = p x 40 ÷ 2 ( 0.8 x 2 ÷ 40 = 0.04 ( ignoring the units )
Double length : R = 0.04 x 80 ÷ 2 = 1.6Ω : / Halve area : R = 0.04 x 40 ÷ 1 = 1.6Ω
Beware the question that asks what happens if the diameter is varied , because is proportional to the diameter squared ,
Doubling diameter will increase the area by four times; halving the diameter will quarter the area ,
Insulation Résistance :
The Insulation between two conductors is considered to act as a ( Series of many high résistance in parallel )
Résistance ( because of the greater number of apparent parallel paths ) Taking the insulation résistance of the original length as R1 ,
Adding an identical extra length is like adding a second R1 in parallel , So :
…….... 1 ……..... 1
….. ─── = ─── so R1 = Rtotal = 100MΩ ( in the first instance )
……....Rtotal … R1
………………………. ……......... 1 ……..... 1……...... 1……...... 2
With double the length ….. ─── = ─── + ─── = ─── so new Rtotal = 50MΩ
…………………………......... Rtotal …... 100 ….... 100…...... 100
Changing the conductor CSA , should have no effect on the insulation résistance for the same value of voltage applied ,
Insulation Resistance Values
0.00 MΩ = dead short :
0.08 MΩ = low insulation resistance fault :
>200 MΩ = healthy circuit :
Notes: ensure all neon's are removed or isolated before commencing testing, as these will make test results appear low during insulation resistance testing.
Notes: Neon's will cause false readings, as will emergency or discharge lighting, so ensure these are all disconnected prior to commencing tests.
Prospective short circuit current (PSC) testing
The prospective short circuit or fault current at any point in an electrical installation is the current that would flow in the circuit if no circuit protection operated and a complete (very low impedance) short circuit occurred. The value of this fault current is determined by the supply voltage and the impedance of the path taken by the fault current. Measurement of PSC can be used to check that protective devices within the system will operate within safety limits and as per the safe design of the installation. PSC is normally measured between the phase and neutral at the DB or at a socket outlet.
The 17th Edition of the Wiring Regulations (BS 7671: 2008) will introduce a number of new requirements for the installation of
RCDs, therefore it is timely to look at the requirements within the17th Edition for verification of RCDs. The continuing effectiveness of these RCDs needs to be confirmed periodically. This article discusses the verification required where RCDs are
used to provide automatic disconnection of supply in the event of a fault and additional protection. It should be stated at this point that the 17th Edition does not introduce any significant changes in the requirements for the testing of RCDs even where they are installed to provide automatic disconnection in the event of a fault ,
Use of RCDs to achieve automatic disconnection in case of a fault :
411.3.2.1 requires (in most cases) that a protective device shall interrupt the supply to a line conductor of a circuit or equipment in the event of a fault of negligible impedance between said line conductor and an exposed conductive- part or a protective
conductor for the circuit or equipment within the appropriate required disconnection time. A disconnection time of 5 seconds
for distribution equipment and final circuits of rating exceeding 32A is permitted by 411.3.2.3. Similarly, a disconnection time of 1 second for distribution equipment and final circuits of rating exceeding 32 A is permitted by 411.3.2.4. : 411.3.2.2 states that the maximum disconnection times of Table 41.1 shall be applied to final circuits not exceeding 32 A. Table 41.1 gives the maximum
disconnection times for final circuits not exceeding 32 A of varying nominal voltages forming part of an
installation having either TN or TT system earthing. These disconnection times may be met by the use of fuses, circuit breakers (formerly known as MCBs) or RCDs. is used to meet the requirements of 411.3.2.2, that is, to provide the required disconnection time, the maximum values of earth fault loop impedance in Table 41.5 may be applied. The maximum permissible earth
fault loop impedances (Zs) to ensure RCD operation for non-time delayed RCDs protecting final circuits not
exceeding 32 A are given in Table 41.5, a new table introduced in the 17th Edition, which is reproduced below.
Where an RCD is employed to achieve the disconnection time required by Table 41.1, it is necessary to satisfy
ourselves that the maximum earth fault loop impedance (Zs) stated for a particular sensitivity of RCD in Table 41.5 is not exceeded in the circuit to which they provide protection. This is in effect the same procedure that we applied in earlier editions where fuses
or circuit breakers were used to achieve the necessary disconnection time and indeed continue to apply for fuses and circuit breakers in the 17th Edition. Regardless of which type of protective device is used to achieve the disconnection times required by Table
41.1, whether fuse, circuit breaker or RCD, there is no requirement to confirm that the required disconnection time can be achieved by testing the protective device. Rather, we confirm that the earth fault loop impedance of the protected circuit does not exceed the relevant tabulated maximum earth fault loop impedance for the type / sensitivity of the protective device intended to provide
the required disconnection time.
Maximum earth fault loop impedance (Zs) to ensure RCD operation in accordance with Regulation 411.5.3 for non-delayed RCDs to BS EN 61008-1 and BS EN 61009-1 for final circuits not exceeding 32 A
( 411.3.2.2 ) 230v
TN- Systems : The maximum disconnection time stated in table 41.1 shall be applied to final circuits Not-Exceeding 32Amp
( 411.3.2.3 )
TN- Systems : in a TN-system, a disconnection time Not exceeding 5sec is permitted for a distribution circuit and for a circuit Not covered by Regulation 411.3.2.2 ,
( the table you require TT-systems 41.5 p-50
Resistivity :
Double cable length – Double Conductor Resistance to 1.6 Ω, but halve insulation resistance to 50 MΩ
Halve . CSA – double conductor resistance to 1.6 MΩ, but insulation resistance is unaffected and remains 100 MΩ
Resistivity :
A twin cable has a Phase to Neutral résistance value of 100MΩ and an individual conductor résistance values of ( 0.8Ω )
Determine the values if the cable ….
(a) was double in length ,
(b) length as the same but the conductor cross-section areas was halved ,
Conductor Résistance
This is a function of the resistivity of the conductor material :
In other words , résistance is directly proportional to length and inversely proportional to area ,
So doubling length or halving the area will both double résistance , try it with some values ,
Let’s say the original length is 40m and the area is 2mm2 . The equation then , is :
0.8 = p x 40 ÷ 2 ( 0.8 x 2 ÷ 40 = 0.04 ( ignoring the units )
Double length : R = 0.04 x 80 ÷ 2 = 1.6Ω : / Halve area : R = 0.04 x 40 ÷ 1 = 1.6Ω
Beware the question that asks what happens if the diameter is varied , because is proportional to the diameter squared ,
Doubling diameter will increase the area by four times; halving the diameter will quarter the area ,
Insulation Résistance :
The Insulation between two conductors is considered to act as a ( Series of many high résistance in parallel )
Résistance ( because of the greater number of apparent parallel paths ) Taking the insulation résistance of the original length as R1 ,
Adding an identical extra length is like adding a second R1 in parallel , So :
…….... 1 ……..... 1
….. ─── = ─── so R1 = Rtotal = 100MΩ ( in the first instance )
……....Rtotal … R1
………………………. ……......... 1 ……..... 1……...... 1……...... 2
With double the length ….. ─── = ─── + ─── = ─── so new Rtotal = 50MΩ
…………………………......... Rtotal …... 100 ….... 100…...... 100
Changing the conductor CSA , should have no effect on the insulation résistance for the same value of voltage applied ,
Insulation Resistance Values
0.00 MΩ = dead short :
0.08 MΩ = low insulation resistance fault :
>200 MΩ = healthy circuit :
Notes: ensure all neon's are removed or isolated before commencing testing, as these will make test results appear low during insulation resistance testing.
Notes: Neon's will cause false readings, as will emergency or discharge lighting, so ensure these are all disconnected prior to commencing tests.
Prospective short circuit current (PSC) testing
The prospective short circuit or fault current at any point in an electrical installation is the current that would flow in the circuit if no circuit protection operated and a complete (very low impedance) short circuit occurred. The value of this fault current is determined by the supply voltage and the impedance of the path taken by the fault current. Measurement of PSC can be used to check that protective devices within the system will operate within safety limits and as per the safe design of the installation. PSC is normally measured between the phase and neutral at the DB or at a socket outlet.
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