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Do we have to install sleeving to all conductors to the modern colours?

gazza — Tue, 26 Mar 2013 00:00:00 +0000

Do we have to install sleeving to all conductors to the modern colours?

gazza Tue, 26/03/2013

Q:When we were replacing the accessories within the property we came across red and black conductors at the sockets and red, blue and yellow conductors at the switches. The earth conductors at all the accessories are bare copper without any sleeving. Do we have to install sleeving to all the conductors to the modern standards?
A: These cable colours were in use until 2006 when the “new” style brown and blue coloured cables.
Firstly if you are to retain part of the existing system, you are required to provide a label at the consumer unit stating that there are wiring colours to two versions of BS7671.
It is permissible to leave the existing red and black conductors providing that they have been correctly identified.
The only requirement to provide new colour sleeving would be if there was some confusion to the identification of the conductors.
Regarding the bare earth conductors at the accessories, any items that you are working on should be provided with green and yellow sleeving. For any accessories that are not part of your work, you should inform your client of the current condition and recommend that an inspection of the existing installation is carried out.
By Frank Bertie NAPIT Group Technical Director

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Can you use a plug in socket-outlet tester to show earthing is satisfactory?

gazza — Mon, 11 Feb 2013 00:00:00 +0000

Can you use a plug in socket-outlet tester to show earthing is satisfactory?

gazza Mon, 11/02/2013

Q: A colleague has stated that to prove socket-outlets work correctly all we need to do is to plug in a socket-outlet tester which lights up to show the earthing is satisfactory. This would save on expensive test equipment and time spent testing, is this correct?
A: The short answer is NO, these socket testers do not confirm the effectiveness of the earthing of the installation. There have been recorded instances where although these socket testers have indicated an earth is present the actual recorded value has exceeded the maximum permitted value for the earth loop impedance.
In this case the fuse or circuit breaker would not have operated within the timescale to prevent an electric shock.

Therefore these socket testers can easily mislead the user into believing a socket-outlet is safe when it is anything but safe.

There is no substitute for following the correct procedures in carrying out electrical testing of the installation. These socket testers when used in the appropriate circumstances can assist the electrical contractor but cannot replace the proprietary test instruments.

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Can I leave open connector joints at the rear of kitchen base units?

gazza — Wed, 06 Feb 2013 00:00:00 +0000

Can I leave open connector joints at the rear of kitchen base units?

gazza Wed, 06/02/2013

Q: We are carrying out alterations to existing circuits and have come across open connector joints at the rear of the kitchen base units, can we leave them in place?
A: No, Live conductors and terminals are required to be provided with an enclosure to ensure there can be no access to those live parts without a means of a key or a tool to prevent the risk of electric shock. There is also a further requirement that the enclosure has to prevent the spread of fire in the case of a fault.

If the cables are Pvc/Pvc or as they are more commonly known Twin & Earth and have the outer sheath removed the single insulated conductors have to be contained within the enclosure.

With any electrical accessory including junction boxes, choc boxes etc, the accessory should be securely fixed to prevent strain on the terminations and cables.

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Copper sheath cable and terminations

gazza — Thu, 11 Oct 2012 23:00:00 +0000

Copper sheath cable and terminations

gazza Fri, 12/10/2012

By Frank Bertie NAPIT Group Technical Director - Question: During a refurbishment of a kitchen we came across copper sheathed cable supplying the accessories. The existing accessories have boxes with metal clamps which are securing the copper sheath cable terminations. The accessories are being relocated and we will have to alter these cables how do we do it or can we replace the cable with twin and earth cable?
Answer: The cable you mention is mineral insulated copper sheathed cable, which consists of normally two copper cores surrounded by a white compound material which acts as insulation, then a copper outer sheath to enclose the cores. Often this type of cable is then coated with an orange pvc sheath to prevent corrosion to the copper sheath.
There are different methods of terminating this type of cable, the one you mentioned is where the cable gland has not been utilised and only the pot/seal have been used, this type of cable requires experience and special tools to correctly terminate.
The cables can relocated but unless you have the necessary experience, it may require someone who is familiar with the cable and its terminations.
Normally when this type of cable is used in a domestic environment it is for a particular purpose, often in heritage style properties there is a requirement for fire rated wiring systems. You would have to check with the local authority or heritage department before making any alterations to the wiring system.

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New and Old Cable Colours - Do I sleeve all old conductors to the new?

gazza — Tue, 09 Oct 2012 23:00:00 +0000

New and Old Cable Colours - Do I sleeve all old conductors to the new?

gazza Wed, 10/10/2012

By Frank Bertie NAPIT Group Technical Director - Q: When we were replacing the accessories within the property we came across red and black conductors at the sockets and red, blue and yellow conductors at the switches. The earth conductors at all the accessories are bare copper without any sleeving. Do we have to install sleeving to all the conductors to the modern standards?

A: These cable colours were in use until 2006 when the “new” style brown and blue coloured cables.
Firstly if you are to retain part of the existing system, you are required to provide a label at the consumer unit stating that there are wiring colours to two versions of BS7671, as sample below.
It is permissible to leave the existing red and black conductors providing that they have been correctly identified.
The only requirement to provide new colour sleeving would be if there was some confusion to the identification of the conductors.
Regarding the bare earth conductors at the accessories, any items that you are working on should be provided with green and yellow sleeving. For any accessories that are not part of your work, you should inform your client of the current condition and recommend that an inspection of the existing installation is carried out.

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What procedures should we take to work safely when working on circuits?

gazza — Wed, 19 Sep 2012 23:00:00 +0000

What procedures should we take to work safely when working on circuits?

gazza Thu, 20/09/2012

Q&A By Frank Bertie NAPIT Group Technical Director - Q: When we are working on circuits which are supplied from an existing consumer unit what procedures should we take to work safely?
A: Firstly, it is a legal requirement that employers must ensure that their employees working on electrical equipment are competent to do so.
Employees must be trained and instructed in the implementation of safe systems of work. This may involve the issue of written procedures, work permits and instructions on the use of locking-off devices, warning notices, approved voltage indicating devices and voltage proving unit.

The main purpose of safe isolation is that the point of safe isolation should be under the control of the person who is going to carry out the work on the isolated circuit or equipment. There are several ways of achieving safe isolations which depend on the type of:

Consumer unit/fuse board supplying the circuit

Overcurrent device used to protect the circuit

Local Isolation for equipment

Therefore, when working on existing circuits you have to be in control of the supply of the circuit, you have to have proved the circuit is dead and cannot become live during the process of your work.

Your options are Lock Off the circuit breaker or remove and retain procession of the fuse, place warning signs at the point of isolation and prove with a voltage tester that the circuit is isolated.

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Grommets and grommet strip - do I need them?

gazza — Wed, 19 Sep 2012 23:00:00 +0000

Grommets and grommet strip - do I need them?

gazza Thu, 20/09/2012

Q&A By Frank Bertie NAPIT Group Technical Director - Q: Do I have to provide grommets in metal back boxes of sockets and switches?
A: Grommets or some other method of protecting the cable from the effects of abrasion from the sharp edges of metal consumer units, enclosures or back boxes.
Care should be taken when the protection is installed as often during the installation process the grommets can move and either fall down or pull up leaving the cables open to damage. This is even more important when using grommet strip as this is more susceptible to movement.

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Can I test my sockets work correctly by using a plug in socket tester?

gazza — Mon, 27 Aug 2012 23:00:00 +0000

Can I test my sockets work correctly by using a plug in socket tester?

gazza Tue, 28/08/2012

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Can I leave open connector joints at the rear of kitchen base units?

gazza — Fri, 24 Aug 2012 23:00:00 +0000

Can I leave open connector joints at the rear of kitchen base units?

gazza Sat, 25/08/2012

Q&A - By Frank Bertie NAPIT Group Technical DirectorQ: We are carrying out alterations to existing circuits and have come across open connector joints at the rear of the kitchen base units, can we leave them in place?
A: No, Live conductors and terminals are required to be provided with an enclosure to ensure there can be no access to those live parts without a means of a key or a tool to prevent the risk of electric shock. There is also a further requirement that the enclosure has to prevent the spread of fire in the case of a fault.
If the cables are Pvc/Pvc or as they are more commonly known Twin & Earth and have the outer sheath removed the single insulated conductors have to be contained within the enclosure.
With any electrical accessory including junction boxes, choc boxes etc, the accessory should be securely fixed to prevent strain on the terminations and cables.

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The New Fuse Standards and the Maximum Zs Tables

gazza — Tue, 26 Jun 2012 23:00:00 +0000

The New Fuse Standards and the Maximum Zs Tables

gazza Wed, 27/06/2012

The New Fuse Standards and the Maximum Zs Tables
by
Bill Allan
Amendment 1 of BS 7671:2008 contains some important changes concerning new fuse standards which took effect in March 2010. These changes can be seen in the maximum earth fault loop impedance (Zs) values in Tables 41.2 and 41.4. (Table 41.3 concerns circuit-breakers) The new fuse standards are:

BS 88-2 which has replaced BS 88-2.2 and BS 88-6.

BS 88-3 which has replaced BS 1361.

(However, fuses complying with these withdrawn standards will be found in existing electrical installations for some years to come.)

Having noted these changes, we are provided with a convenient opportunity to consider how these maximum Zs values in Tables 41.2, 41.3 and 41.4 are obtained.

This article will look at the maximum Zs values for BS88-3 fuses in Table 41.4(b) as an example of how these maximum Zs values are obtained.

The formula
The maximum earth loop impedance, Zs values are arrived at using the formula in Regulation 411.4.5:
Zs x Ia ≤ Uo
where:
Zs = the earth fault loop impedance in ohms
Ia = the current in amps which causes the protective device to operate within the time specified in Table 41.1 (or, if appropriate, within the 5 second disconnection time specified in Regulation 411.3.2.3 for TN systems).
Where an RCD is used, the current Ia is the rated residual operating current which provides disconnection within the time specified in Table 41.1 (or, if appropriate, within the 5 second disconnection time specified in Regulation 411.3.2.3 for TN systems).
U0 = the nominal voltage to Earth in volts (ie. 230 V for standard a.c. single-phase and three-phase circuits)

Appendix 3
The value of Ia can be obtained by reference to the appropriate time/current characteristic of protective devices and RCDs in Appendix 3 of BS 7671.

Example of 20A BS88-3 fuses
Let us assume we have a 230V, TN, a.c. system with overcurrent protection provided by 20A BS 88-3 fuses for a final circuit not exceeding 32 A.
Table 41.1 requires that disconnection of the circuit takes place within a maximum time of 0.4 seconds.
To calculate the maximum earth fault loop impedance Zs which will ensure that 20A BS 88-3 fuses will disconnect the circuit within 0.4 seconds, we must use the formula in Regulation 411.4.5 shown above, but transpose it to make
U0 the subject. This gives:
Uo
Zs ≤ Ia
U0 = 230V.

To obtain Ia, we turn to Fig. 3A1 in Appendix 3, which gives the time-current characteristics for BS 88-3 fuses.

The line representing the 20A fuse gives us the value of current but it’s much easier to use the table at the top right of the page. (If you need the practice, you could use the 20A line to confirm the value obtained in the table) From this table we can see that the value of current which causes a 20A BS 88-3 fuse to operate within 0.4 seconds is 113 A. Turning again to the formula:

Uo
Zs ≤ Ia
230
Zs ≤ 113
Zs ≤ 2.04 ohms.
As the required disconnection time is 0.4 seconds, we turn to Table 41.2(b). Here we find that the maximum Zs value for 20 A BS 88-3 fuses is 2.04 ohms.
BS 3036 and BS 1362
The other fuse standards in Tables 41.2 and 41.4 – BS 3036 and BS 1362 – remain at the same values as in BS7671:2008 , so the corresponding values of maximum Zs are also unchanged.
The same process can be followed for any of the protective devices listed in Tables 41.2, 41.3 and 41.4 to obtain the Ia value from Appendix 3 to determine the calculated values Zs.

Appendix 3
Reference to the appropriate time/current characteristic of protective devices and RCDs in Appendix 3 of BS 7671 can also be utilised for the discrimination between protective devices.

Conclusion
From the maximum Zs tables and Appendix 3, we can see the relationship between the current required to operate the protective device and the earth loop impedance values.

The value of the earth fault loop impedance path Zs determines the current that will flow in the event of a fault to Earth. This value of current in turn determines the time that a particular protective device will take to operate (ie. automatic disconnection). Therefore the Zs value of a circuit is an essential characteristic in the protective measure known as automatic disconnection of supply (ADS).

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