Page updated 10th December 2010
Welcome to our 2391 Inspection Testing of Electrical Installations free advice page.
The following is a list of tips, advice and guidance to help the 2391 student prepare for taking the exam
Correct use of terminology:
The student must be aware of the correct use of terminology when answering questions, marks will be lost for incorrect use, some common examples include:
Using the term 'live' instead of 'phase'. Remember a live conductor includes the neutral also
Using the term 'completion certificate' instead of Electrical Installation Certificate
Using the term 'electrical inspection certificate' instead of Electrical Installation Certificate
Using the term 'initial' inspection certificate' instead of Electrical Installation Certificate
Using the term 'minor works certificate' instead of Minor Electrical Installation Works Certificate
Using the term 'Periodic Inspection Certificate' instead of 'Periodic Inspection Report for an Electrical Installation'
Incorrect use of the terms used to describe the 'Earthing Conductor' and Main Equipotential Bonding Conductors' For example using the incorrect term 'Main Bonding Conductor' instead of 'Earthing Conductor' and using the incorrect term 'Earth Bonding Conductor or even Bonding Conductor' instead of Main Equipotential Bonding Conductor
Incorrectly describing the 'Continuity of protective conductors' or 'Continuity of Ring Final Conductors' test as 'Open Loop' test when describing how to obtain R1 + R2
Incorrectly referring to 'Inspection Schedule' instead of 'Schedule of Inspections'
Incorrectly referring to 'Schedule of Tests' or Test Results Schedule instead of ' Schedule of Test Results'
It may sound petty, but you must learn to use the correct terms or you will lose marks - All the available past exam papers and the periodic inspection report check-list describe the correct terms to use
When requested the documentation to complete during an initial or periodic inspection, many candidates will mention the Electrical Installation Certificate or the Periodic Inspection Report but forget to mention the 'Schedule of Inspections' and Schedule of Test Results'
Many of the latest written exam papers include the requirement to complete an Electrical Installation Certificate or Periodic Inspection Report along with the accompanied 'Schedule of Inspections' and Schedule of Test Results' Often the question only asks the candidate to complete the schedule of test results for say a new ring final circuit. Many candidates waste valuable time and lose marks by completing the schedule of test results for every circuit given in the specification in part B of the question paper.
BIG TIP: READ THE QUESTION CAREFULLY:
Many candidates continue to get the value of Zs wrong for the circuit in question in part B of the exam. Check out my sample exam papers with answers December 2006 to see the correct way to tackle these type of questions.
A common error when completing questions involving schedule of test results is to forget to indicate functional tests have been performed and found satisfactory/unsatisfactory. To tick the tick box when completing details of ring final circuits, ensuring to indicate continuity of ring final conductors have been performed.
Failure to record the type of earthing system, i.e. TN-S, TN-C-S, TT,
Failure to record the value of Ze, PFC, Nominal voltage, Nominal frequency, are common errors.
A very common question is to explain in detail how to perform an insulation resistance test, often on a lighting circuit. Candidates regularly fail to state the instrument used which is an 'Insulation Resistance Ohm-meter' (Not a Megger! You will not gain any marks if you answer a Megger) Many candidates fail to identify the test voltage required for typical 230/240 volt installation which is 500 volts and the acceptable test value which is a minimum of 1.0 Meg-ohm. City and Guilds usually deliberately pick a lighting circuit that is stipulated as having two way switching, just to see if the candidate mentions to test the strappers in BOTH positions.
Failure to mention testing the insulation resistance of both strappers will lose you marks. Be warned!
Candidates regularly make mistakes when answering RCD questions. Often the question or specifications usually given in part B will make reference to a specific type of RCD for example a 30 mA RCD. Candidates are then asked to state the actual test current applicable to test this type of RCD. Candidates regularly incorrectly state the answer as x1/2 x1 and x5 instead of x1/2 = 15mA x1 = 30mA and x5 = 150mA
A frequent question which often occurs requires the candidate to explain how to perform a continuity test on a ring final circuit and to explain how R1 + R2 for the circuit is obtained. Many candidates regularly continue to get this procedure wrong check out the Periodic Inspection Report check-list for a thorough explanation on how to perform these tests.
New information 12/07/2007 City and Guilds are now starting to ask questions based in IT systems. Having checked in the 2391 syllabus it states the candidate should be able to differentiate between the requirements for testing earth electrodes for RCD protected IT systems and electrodes for transformers and generators. IT systems are so rare in the UK, only a small group of sparkies will ever come across one. The installation arrangements for IT systems are similar to TT systems except the earthing arrangement is completely different. The IT system can have no earth at all, or an earth which connects through a current limiting resistance called the 'Earthing Impedance' Due to the complete absence of an earth or due to the earthing impedance the normal EEBADS method of protection will not provide sufficient protection against electrical shock. Because of this IT systems are not allowed in the general public supply system in the UK.
In the February 2007 exam paper C&G asked the candidates to state an earthing system where 'an Earthing Impedance' would be used. The answer of course you now know is an IT system. We will have to wait and see if C&G ask any more questions about this very rare system ?????
New information 12/07/07 C&G are also starting to ask question based on the 'Statutory' requirements relating to inspection and testing see my February 2007 paper Q25 d). Now C&G have always asked candidates to be able to name the statutory documents e.g. 'The Electricity at Work Regulations 1989' and the 'Health and Safety at Work Act 1974' etc. But never to my knowledge what is contained in those regulations. Until now ???. I can only suggest if you want to be fully prepared for the 2391 exam you get hold of a copy of the guidance booklet titled 'Memorandum of Guidance on the Electricity at Work Regulations 1989' available probably via order from any regular book store the ISBN number is 9780717662289 published 10/04/07, cost £11.99. C&G are certainly making their exam papers much harder than in the past Phew !!!
In the March 2007 paper C&G point out many exam candidates still cannot list the correct sequence of tests for a given scenario. One such scenario involves the testing of a lighting circuit, many candidates just listed the typical sequence including continuity or ring final circuit conductors, clearly they received 'Nil Pwa' for their effort
Many candidates could not correctly perform calculations involving the 'rule of thumb' or incorrectly applied the rule to the 'Measured' values instead of the 'Tabulated' values
On questions regarding insulation resistance, some candidates thought 0.00 Meg-ohms was an open circuit. Many candidates could not correctly describe the procedure for performing an external phase-earth fault loop impedance test. C&G are now requesting much more detail to such questions as well as RCD testing, requesting not only details of how to perform an RCD test but circuit preparation before hand as well as reasons why circuit preparation is required.
October 2007 exam, part B scenario gave many candidates a difficult time. Here you were presented with a TN-C-S system installed in a domestic property, and an underground supply cable is being used to supply an external outhouse. However the electricity supply company will not allow you to use their means of earthing for the outhouse. So how do you provide a means of earthing for the outhouse? Where do you earth the supply cable? What checks must you make on the underground supply cable? Why can't you use the main house TN-C-S system as a means of earthing the outhouse? Many candidates have written to me seeking clarification on the part B scenario questions.
Should you have any questions you can contact me via my e-mail link at
Best wishes in your studies