Impedance Earth loop

The total earth loop impedance for ready operation of the largest rated excess current protective device relied upon for earth leakage protection. 

When cables are to be sized for a particular project with regard to their short-circuit performance it is necessary to consider the let-through current of the protective device in the circuit e.g., fuse, circuit breaker. It is also necessary to determine whether the consnmer has fixed equipment such as a motor, or temporary equipment such as a portable tool plugged into a socket, because this establishes the minimum time dnration. This aspect is described in more detail when the earth-loop impedance is being considered, see sub-section 9.4.3.6. 

Figure 9.9 Circuit diagram for the earth loop impedance.

Table 9.34. Earth loop impedance results for the worked example with braided armouring...

Comparing the tabulated results above with those for the fuses in d) shows that all the cables have an earth loop impedance much greater than that permitted by the fuse, by a ratio of approximately 10 1. 

If a 50 mm cable and a fuse rating of 125 amps are chosen as recommended in g) then the circuit earth loop impedance is still too high by a ratio of about 1.65 1. Hence an earth leakage circuit breaker should still be used for this motor circuit. The hazardous shock voltage is still too high. 

Table 9.36. Limiting values of earth loop impedance when MCCB ...

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

A key factor in the design and choice of earth continuity conductors, e.g. cable armouring, bonding straps, and fault current protective devices is the earth loop impedance . This is especially the case for solidly earthed low voltage systems, whether they be three-phase, single-phase or even direct current systems. 

The earth loop impedance is the total impedance seen by the source of voltage in a faulted circuit which involves the earthing conductors. Eigure 13.10 shows the situation for a three-phase cable supplying a load such as a motor or static load. 

Figure 13.10 Earth loop impedance of a three-phase circuit.

Figure 13.11 Equivalent circuit of the earth return paths in the earth loop impedance circuit involving a cable and its armouring. This is an interpretation of BS7430 Clause 3.13.

In order to safeguard against electric shock at the far end of the cable, where the AC root mean square voltage may exceed 50 V, the earth loop impedance must be limited to a particular value. This value is such that the fault current should only be passed by the protective device at the supply for a specific period of time, i.e. to satisfy the /-squared- criterion given in sub-section 13.1.1. The correlation of loop impedance, current and time varies with the type of protective device, e.g. fuse. 

The use of a 50 N relay is recommended in relation to the maximum earth-loop impedance allowed for the particular consumer. This will be a function of the motor rated power, the route length and the type of armouring used for the motor power cable. 

When it is required to provide the very best protection from eiectric shock and fire risk, earth fauit protection devices are incorporated into the instaiiation. The object of the reguiations concerning these devices (411.3.2 to 411.3.3) is to remove an earth fauit current very quickiy, iess than 0.4s for all final circuits not exceeding 32 A, and limit the voltage which might appear on any exposed metal parts under fault conditions to not more than 50 V. They will continue to provide adequate protection throughout the life of the installation even if the earthing conditions deteriorate. This is in direct contrast to the protection provided by overcurrent devices, which require a low-resistance earth loop impedance path. 

If the inspecting engineer has grounds to question the soundness and quality of these joints then the phase earth loop impedance test described later in this chapter should be carried out. 

Using a line earth loop impedance tester, test between the incoming line conductor and the earth electrode. 

The test will, in most cases, be done with a purpose-made line earth loop impedance tester which circulates a current in excess of lOA around the loop for a very short time, so reducing the danger of a faulty circuit. The test is made with the supply switched on, and carried out from the furthest point of every final circuit, including lighting, socket outlets and any fixed appliances. Record the results on a schedule of test results. 

The phase voltage at the substation transformer will be a little higher than 230 V to allow for the inevitable voltage drop in the distribution cables. In urban areas, the line/neutral and the line/earth loop impedances will be comparable and will probably be only a small fraction of an ohm, whereas the victim s hand-to-hand impedance will be in the order of 2000 ohms. Under these circumstances the effects of the circuit impedances can be ignored. The victim s touch voltage will be about 230 V and, for a total body impedance of 2000 ohms, the shock current would be 230/2000 = 0.11 A. This is high enough to cause ventricular fibrillation in many people should the current flow for about 0.5 s. 

In circumstances where sufficiently low values of earth loop impedance cannot be achieved, such as on TT systems, the disconnection device may need to be an RCD, as described in the next section. 

There has been a new addition to the information to be provided by distributors free on request to low voltage consumers. The 1988 Regulations stipulate that they must provide a written statement of maximum prospective short circuit current maximum earth loop impedance and the type and rating of supplier s fusible cut-out or switching device. The new Regulations add to this the type of earthing system (TT, TN etc). 

When calculating the earth loop impedance, Zs, that part of it which consists of metalwork used as a protective conductor, e.g. conduit, trunking and switchboard metalwork, is normally ignored because its resistance is usually negligible. Where the earth fault loop impedance exceeds the values... 

If the earth loop impedance is too high for overcurrent protection to operate within the prescribed time, Regulation 413-02-04(i) permits this time to be exceeded provided local supplementary equipotential bonding is used to meet the requirements of Regulations 413-02-27 and 28 and the fault persistence does not cause damage. Again, the intention is to minimise the potential differences between touchable metalwork items on the occurrence of an earth fault. As an alternative. Regulation 413-02-04(ii) allows the use of RCD protection. 

Although the electricity supply companies have converted a substantial number of TT systems to TN-C-S systems, some TT systems are still in use, mostly in rural areas where it is often difficult or impracticable to obtain and maintain a low earth loop impedance. In these cases, the overcurrent protection should be supplemented by RCD protection as required in Regulations 413-02-18 to 20 and 471-08-06. 

Regulation 542-0 l-07(i) means that the connections to earth must be effective and the earth loop impedance low enough for the protective devices to operate within the prescribed time in the event of an earth fault. Regulations 542-01-08 and 542-02-03 deal with electrolysis and corrosion. The electrolysis in Regulation 542-01-08 is probably that which occurs in d.c. supply systems where earth leakage can cause corrosion from electrolysis at the positions where the current enters or leaves metallic parts. This problem occurs in d.c. traction systems and certain types of electrochemical plants, and is often associated with cathodic protection systems which impress d.c. currents in the ground. 

The difficulty about the calculation method is in determining the prospective fault current when the supply is derived from a supply company s LV network. Changes to the network and to the installation can affect the earth loop impedance and the prospective fault current. On the whole it is probably better to use Table 54G than do the calculation and employ a smaller section conductor at what may be a small cost saving, only to find later that it was a false economy. 

If the supply system is PME, the supply company is precluded from providing an earthing terminal for outdoor sites because there is no equipotential zone, so the contractor has to provide his own earth. He may well then find that the earth loop impedance is too high for operation of the excess current protection, on the occurrence of an earth fault, within the specified time and he will have to resort to earth leakage protection using an RCD. 

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