n appreciable amount of time and nervous energy may be saved by systematically planning your testing programme. The basis of this planning will be to regard an installation overall as being constructed of smaller parallel and series interconnected installations.
For example, sub-distribution boards are connected in parallel with each other but in series with the main distribution panel.
Or in other terms, current flow in each of the sub distribution boards will not affect the others, but will change the current demand of the main distribution board.
This arrangement is shown in the schematic diagram below.
Distribution boards B, C, and D are in parallel with each other and connected in series with distribution board A.
Tests of external P-E loop impedance will take place at A, B, C and D
At positions b,c and d the loop impedance will include that of the connected distribution circuit.
But this is of no
Therefore, it means that this installation will have four values of Ze. Measurements and evaluation will be made locally, for each distribution board.
For example, if a particular circuit connected to distribution board B had a maximum
R1 + R2 value of 0.7 Ω and the external P-E loop impedance of the DB was 0.2 Ω, the uncorrected impedance would be 0.9 Ω. Assuming a short circuit temperature correction factor of 1.2, the corrected loop impedance will be,
0.7 x 1.2 + 0.2 = 1.04 Ω This value would now be evaluated by reference to table 41B1, 41B2 or 41D, as appropriate.
It should be noted that the Ze of the sub distribution board will be the Zs of the supplying distribution circuit.
All other tests should be conducted on the basis of an individual distribution board forming an electrical installation, having unique internal and external characteristics.
A
consequence: as far as the external circuit is concerned it, consists of resistance and reactance - the components of opposition have the same effect regardless of location and form.
A
B C D
Fig.46
Inspection and Testing. A.W.Croucher 2 004 page 72
Adherence to this principle will mean that data sheets will have to be compiled on each of the individual distribution boards - don’t try to compile a report on a large installation on a single sheet - practical experience has indicated that there really is no other way.
As previously stated before an installation is tested and inspected a pre-planning stage should be completed. This pre-plan includes a survey of the installation component parts and will include a determination of the means of control and overcurrent protection.
Additionally thought should be given to tools, instruments, access equipment, temporary lighting extension leads etc.
If the contents of distribution boards are a mystery - before any meaningful inspection and testing can be conducted - distribution board schedules must be completed, an example of which has previously been presented.
Shown Overleaf are suitable forms on which this pre-planning may be organised
Inspection and Testing. A.W.Croucher 2 004 page 73
Preplan check list
Have you arranged for permission to enter all of the relevant parts of the building? Don't forget the intake room may not be under the control of the client.
Do you have permission to shut off supplies to those parts of the building under test? Don't forget about the possible need for computers and Fax machines be running continuously.
Are all of the sub distribution board cupboards accessible, do they need individual keys to gain access?
Ensure that as a minimum the following items are considered before arriving on site.
Item Notes
Access equipment Steps - tower scaffolding
Keys Access will be required to all relevant part s of
the building
Tools Pliers, screwdrivers- manual and electric,
spanners etc.
IR tester One for each team member
P-E loop impedance tester One for each team member
RCD tester One for each team member
Polarity tester One for each team member
Voltage tester One for each team member
Torch One for each team member
Extension lead Needed for p-e loop testing
Temporary lighting For those dark areas needing inspection
Clamp meter For measuring maximum demand
Spare batteries For the insulation resistance tester AA size Documentation For the direct recording of measured values
Pad of paper For rough work
Pens - pencils Particularly pencils – needed for completion of NIC PIR forms in draft.
Calculator For calculating P-E loop values etc.
Adhesive labels For attaching to points when the parent circuit has been identified.
Pair of radios Can save an enormous amount of time when working in pairs distant from each other.
Digital camera For enhancing your presentation. Photographs of damage, malpractice and installed systems.
Inspection and Testing. A.W.Croucher 2 004 page 74
SECTION 15
TESTING PORTABLE AND TRANSPORTABLE EQUIPMENT
One of the most relevant and important tests is the PRELIMINARY VISUAL INSPECTION. In practice, approximately 80 % of all equipment defects are found at this important stage.
Hazards such as loose cord grips, plugs incorrectly fitted, unsafe cable joints, incorrectly rated fuses etc. should all be discovered and correct at the physical inspection stage before electrical tests commence.
The equipment to be tested will normally be constructed in one of the basic classes, designated Class 1, 2 or 3.
CLASS ONE equipment will be of the conventional insulated and mechanically protected construction. The enclosure will be earthed via a protective conductor contained within a three-core flexible cord.
CLASS TWO equipment provides two levels of protective insulation. The first protective barrier is the functional or basic insulation, while the outer protective layer will comprise either an insulating internal lining to a conducting enclosure or form the enclosure itself.
CLASS THREE equipment will operate at either ELV or a reduced voltage supply such as 110 V centre tapped earth.
Tests of protective conductors.
This test must be performed at a voltage not exceeding 12 V, with a test current at least 1.5 times that of design current with a maximum of 25 A. The test current will be applied for 5 seconds.
The resistance should not exceed 0.1Ω, excluding any resistance provided by the plug.
If the appliance under test is of low current demand, BS 415 and BS 4533 permit a combined resistance of 0.5Ω.
Tests of insulation resistance.
BS 2754 requires a minimum insulation resistance of 2 megohms between live conductors and earth.
A high voltage or " flash " test may also required. Test voltages will vary according to the equipment being tested, the most stringent being 2.1 kV
BS 3456 (household appliances) allows a leakage current of 0.75 mA/kW with an overall maximum of 5 mA.