The purpose of this test is to ensure that:
•
The cables form a complete ring.•
There are no interconnections.•
The polarity is correct on all socket outlets.When this test is carried out correctly it also gives you the R1and R2value of the ring and identifies spurs.
Table 8A in the On-Site Guide provides information on final circuits for socket outlets. This table states that a ring circuit is to be wired in 2.5 mm2phase conductor and 1.5 mm2CPC as a minimum size. This type of circuit is an A1 ring and should be protected by a 30/32 amp overcurrent protective device.
Complete ring circuit
A test must be carried out on the conductors to verify that they form a complete loop. If it is found that they do not, overloading of the cables could occur. In installations where more than one ring circuit has been installed, it is possible for the ends of the ring to become muddled, resulting in the circuits being supplied through two protective devices.
The whole point of a ring circuit is that it can be wired in small CSA cables but carry a reasonably high current, this is because we have two 2.5 mm2 cables wired in parallel (Regulation 473-01-06). If we look at Table 4D5A in BS 7671, the value of current that 2.5 mm2cable can carry is 20 amps in the worst type of conditions.
If we use two of these conductors in parallel, we will have a total current carrying capacity of 40 amps. As one of the jobs of the protective device is to protect the cable, this situation will be fine because the protective device is smaller than the total current carrying capacity of the cables in parallel.
Broken ring circuit
If, however, we found the ring to be broken, the protective device could not do its job as it is rated at 32amps and the cable is rated only at 20amps. Hence overloading!
Interconnections
Occasionally a situation will be found where there is a ring within a ring, in other words the ring is interconnected.
32 amp MCB 20 amp
MCB 16 amp
MCB 6 amp
MCB
Neutral BarEarth Bar ON OFF
Complete ring circuit
32 amp MCB 20 amp MCB 16 amp MCB 6 amp MCB
Neutral BarEarth Bar ON OFF
Broken ring circuit
32 amp MCB 20 amp MCB 16 amp MCB 6 amp
MCB
Neutral BarEarth Bar ON OFF
Interconnected ring circuit
This situation, as it is, will not present a danger. However, it will make it very difficult for a ring final circuit test to be carried out as, even if the correct ends of the ring are connected together, different values will be found at various points of the ring. If one loop is broken, a test at the consumer’s unit will still show a complete ring. It will not be until further tests are performed that the interconnection/broken loop will be found.
Polarity
Each socket outlet must be checked to ensure that the conductors are connected into the correct terminals. Clearly if they are not, serious danger could occur when appliances are plugged in.
It could be that phase and neutral are the wrong polarity; the result of this is that the neutral would be switched in any piece of equipment with a single pole operating switch.
If the live conductors and CPC are connected with reverse polarity, then the case of any Class 1 equipment could become live and result in a fatal electric shock.
Performing the test
The instrument required is a low resistance ohm meter set on the lowest scale, typically 20Ω. Be sure to zero the instrument or subtract the resistance of the leads each time you take a reading.
This is a dead test! Safe isolation must be carried out before working on this circuit
STEP 4
Test between ends of neutral conductor. This value should be the same as the phase conductor
resistance as the conductor must be the same size (see Note 1).
Ends of phase conductor
1kV V
500V
250V OFF
SET UP kΩ MΩ
Ω
Instrument set to Ω for whole test.
Ends of neutral conductor
STEP 1
Isolate circuit to be tested.
STEP 2
Identify legs of ring.
STEP 3
Test between ends of phase conductor and note the resistance value.
STEP 5
Test between the ends of the CPCs. If the conductor size is smaller than the live conductors (as is usually the case when using twin and earth cable), the
resistance value will be higher (see Note 2); make a note of this reading.
STEP 6
Join P of leg 1 to N of leg 2.
Test between N of leg 1 and P of leg 2. The measured resistance should be double that of the phase conductor.
Ends of CPCs
Resistance double that of phase conductor
P2 P1 joined to N2 N1
STEP 7
Join N of leg 1 to P of leg 2 together (leaving N2 and P1 joined).
Test between joined ends.
The measured value should be of test between N of leg 1 and P of leg 2.
14
STEP 8
Leave the ends joined.
Test between P and N at each socket outlet, the resistance should be the same at each socket (see Note 1).
A higher reading should be investigated, although it will probably be a spur it should be checked as it may be a loose connection (high resistance joint).
Resistance value the same at each socket
N P Test at
each socket outlet Test between
joined ends P2 joined
to N1
P1 still joined to N2 Resistance 1/4
of that tested between N1 and P2
STEP 9
Disconnect the ends and repeat the test using phase and CPC conductors (see Note 3).
Phase CPC joined to phase CPC
P2 joined to CPC 1
P1 joined to CPC 2
CPC P Test at each
socket outlet
The highest value (which will be the spur) will be the R1and R2value for this circuit.
Example 3
Let’s use a 2.5/1.5 mm2twin and earth cable 22 metres long. If we look at Table 9A in the On-Site Guide we will see that the resistance of a copper 2.5 mm2conductor has a resistance of 7.41 mΩ per metre.
The resistance of the phase conductor will be:
Divide the largest conductor by the smallest to find the ratio of the conductors (how much bigger is the larger conductor?).
The 2.5 mm2conductor is 1.67 larger than the 1.5 mm2conductor; therefore, it must have 1.67 less resistance than the 1.5 mm2conductor.
2.5 1.5 1.67 7.41 22
1000 0.163Ω Notes
1. If with ends connected (P1/N2 and P2/N1) a substantially different resistance value is measured at each socket outlet, check that the correct ends of ring are connected. A difference of 0.05Ω higher or lower would be acceptable.
2. In a twin and earth cable the CPC will usually have a resistance of 1.67 times that of the phase conductor as it has a smaller cross-sectional area.
3. When phase and CPC conductors are not the same size a higher resistance value will be measured between Phase and CPC than Phase and neutral. It will also alter slightly as the measurement is taken around the ring, the resistance will be lower nearer the joined ends and will increase towards the centre of the ring. The centre socket of the ring will have the same resistance value as the test between the joined ends.
4. If the circuit is contained in steel conduit or trunking parallel paths may be present, this would result in much lower R1 R2resistance values.
5. Some certificates may require rnto be documented. This is the resistance of the neutral loop measured from end to end.
If we now multiply the resistance of the phase conductor by 1.67:
0.163 1.67 0.27 Ω this is the resistance of the 1.5 mm2conductor.
We can check this by looking at Table 9A of the On-Site Guide once again, and we can see that the resistance of 1.5 mm2copper is 12.10 mΩ per metre. Therefore, 22 metres of 1.5 mm2copper will be:
As a final check, if we look at Table 9A of the On-Site Guide for the resistance of a 2.5 mm2/1.5 mm2cable, we will see that it has a resistance of 19.51 mΩ per metre, and that 22 metres of it will have a resistance of:
The resistance value of the 2.5 mm2is 0.163Ω; and the resistance value of the 1.5 mm2is 0.266Ω.
If we add them together: 0.163Ω 0.266Ω 0.429Ω. Finally, 0.429Ω is the resistance of our 2.5 mm2/1.5 mm2measured as one cable.