Impedance Armouring

Earlier studies of adsorption on LL interfaces involved phospholipids (28-31). To investigate possibilities for study of a less defined material (32), we chose a protein as the adsorbing material bovine serum albumin (BSA) with a molar mass of 69,000 (CRG-7, Armour Pharmaceutical Company). The preparation is known to contain a certain amount of fatty acids. The same starting material was used throughout the measurement. The aqueous protein solutions were prepared fresh from a dry sample before each experiment. The effects of BSA on the imaginary and real impedance and interfacial capacitance of a water-nitrobenzene interface are discussed under varying BSA concentration, temperature, and pH. 

Steel wire armour as opposed to steel wire braid has lower electrical impedance for a given length of cable. This is an important benefit in networks that are solidly earthed at their power source. Some special applications that require as low an impedance as is practical to achieve in the cable have some of the armour wires replaced by copper wires. Hence the parallel circuit consisting of the steel and copper wires has a lower total impedance than the steel wires on their own. The impedance of the armouring, with or without the copper wires, is predominantly resistive and so the inductive reactance at the power frequency can therefore be ignored. 

Replace the steel wire braid armour with round steel wires (GSWA) and reduce the metallic return path impedance Zmr to 0.1 ohm, and compare the effect on the hazardous shock voltage. 

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

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. 

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.

The driving voltage is the phase voltage. The sonrce impedance Zs is fixed and is usually that of the upstream supply cables and transformers, or generators (or the output impedance of supplies such as a UPS). Zs can often but not always be neglected. The cable conductor impedance Z is easily calculated from the cable data for one phase conductor and its route length. Similarly the armouring impedance Z can be calculated from the data, which is predominantly resistive for most types of power cables. For typical cable data see the tables in Chapter 9. 

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. 

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