Determining Zs

Zs is the total value of impedance measured at the furthest point along the circuit. It consists of Ze, the external part, and R1+R2, the internal part. (Zs = Ze+R1+R2). As in Fig.1, Zs broken down would consist of Z1+Z2+Z3+Z4+Z5.














Fig. 1




The reason the loop impedance Zs value is required is the same reason as that for requiring Ze, to determine that the earth fault path will have a low enough impedance to enable a large enough fault current to flow to operate the protective device within the specified time. The earth fault loop impedance should be measured for every distribution circuit and final circuit having EEBAD as its method of protection against indirect contact. The measurement of Zs can be determined using a loop impedance tester and should be taken at the point or accessory electrically furthest from the supply to the circuit. Values of Zs may be obtained wholly by the use of a loop impedance tester, or by calculation using the values of R1+R2 obtained previously and adding them to the value of Ze.

When testing a ring circuit the value of Zs is obtained simply by plugging in a loop impedance tester at each point on the ring and recording the result. Lighting circuits or general radial circuits will require the removal of the last lighting fitting or accessory on the circuit and the loop measured between the phase and CPC of the cable. It is not acceptable to measure Zs on this type of circuit using a local 13a socket for the supply source and to use a wandering lead to connect to the metalwork or CPC. The loop impedance then being measured is through the local ring circuit phase conductor to the lighting or radial circuit CPC. This would in all probability lead to a lower than normal loop impedance result for the circuit. This also applies to any other circuits, i.e. a motor circuit for instance, the loop must be tested using the motor cabling itself, not by using a long 240v lead with the loop tester plugged into it and a wandering earth lead onto the motor casing.

Confusion regarding Ze and Zs can arise where a system consists of a supply source, MCCB panel board plus further distribution boards as in Fig.2. For the MCCB incomer the supply source Ze is used for the installation in general. At the DB1 position, the MCCB test results may indicate a Zs to DB1 of (say) 0.124 ohm, at DB1 the value of Ze (Known as Zdb), will be the value of Zs to that board from the MCCB board, i.e. 0.124 ohm. At DB2 position, the value of Ze (Zdb) will be the value of Zs to DB2 from DB1.

Example distribution schematic.



















When Ze and Zs are known, R1+R2 for the cable feeding DB1 can (in theory) be calculated:


However, this method is unreliable due to the likely presence of parallel earth paths. A valid reason for determining the value of R1+R2 is to prove the continuity of the CPC. This cannot be obtained with any degree of certainty by assuming that Zs - Ze = the R1R2 value. A satisfactory reading could easily be obtained with the CPC disconnected and the earth connected to the structural steelwork! Remember also, should a loop test be carried out with the CPC broken or not connected in some way, a dangerous voltage may appear on the metalwork of the circuit being tested.

Where values of R1+R2 and Ze are known, the final value of Zs for the circuit can be calculated, simply adding the values of R1+R2 and Ze together results in the final circuit Zs value. This method of calculating Zs is usually used only at the design stages, when the installation has been completed, where possible, actual measured values should always be obtained. However, this method is useful where an RCD is in circuit and the use of a loop tester would cause the RCD to trip.

Once the value of Zs has been determined, we need to do something with it other than just write it down on the Schedule. As with Ze before, we need to prove the result is of a low enough value to be suitable for the circuit design.

Typical values of maximum Zs for a type C mcb to BS EN60898 from table 41B2 of BS7671 are as follows;


These values are the maximum permissible when the conductors are at their normal operating temperatures. Using a rule of thumb method, assuming when tested the cable and ambient air temperature is 20 deg C, the values should be reduced by 20%, or the values multiplied by 0.80.

Therefore, if the measured value of Zs for a circuit has been recorded at 0.8 ohms and the mcb is 20a, the maximum allowable measured Zs impedance, using the maximum Zs from the table above is;


Similarly, with an mcb of 6a, a measured value Zs of 3.5 ohms, the maximum value allowed would be;


This circuit would therefore fail, and would need to be re-designed using possibly a type B mcb which has a higher max. Zs value of 8 ohms (using the rule of thumb method, this changes to 8 x 0.80 = 6.4 ohms) or the phase and/or the CPC increased in size, or a parallel CPC run together with the main cable run. Obviously, any changes from a recorded design would need to be verified with the engineer concerned as, as in this example, the change to a more sensitive mcb could result in nuisance tripping due to high starting currents.
Circuits that fail by a small margin would still operate the mcb concerned, but the mcb would operate in a time period exceeding the 0.4 or 5 seconds maximum time allowed to give satisfactory shock protection.

During periodic testing, engineers sometimes come across the old 'Type B' plug in type of circuit breaker, commonly made by Wylex. These breakers were designed to easily replace the older type of rewireable fuse type BS 3036. They consisted of a 10mm diameter push button to set the breaker, and a smaller 4mm button to trip it. This type of mcb does not comply with current legislation, being manufactured to the old standard BS 3871 between 1968 and 1980. The maximum permissible fault current on these breakers was between 1 and 2ka only; this is much less than the current BS 60898 mcb's in use today, (typically 9-10kA). For overload protection these mcb's may be satisfactory, their characteristics being similar to the current BS 60898 standard. However, under fault conditions the mcb may fail due to the low kA rating. Where engineers come across these I would suggest we recommend they be changed for the BS 60898 type of breaker.




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DB2 Feed cable
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Z1
REC Neutral
Z2 REC Supply Phase
Source
Z5 Earth Path
Transformer
Z4 (R2, CPC)
Fault
Load
Z3, (R1, Phase Cond)
Determining Zs.
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Page 4 of 7
Tony Spry's Electrical Engineering Pages.
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