MCCB Coordination with wires

4. 4. 1 Protection of wires

MCCB must interrupt the accident current so that the wire temperature rise, caused by the Joule heat generated at the wire in an accident, stays lower than the tolerable value.

The wire’s tolerable temperature is determined by the wire’s insulation material. This is the limit current that does not degrade the insulation sheath, and is not an isolated value. Even if the wire conductor’s temperature increases for a short time, the insulation material will not degrade, and a high temperature can be tolerated. Therefore, the wire’s tolerable temperature can be divided into three categories: continuous use, short-time use, and use under short-circuit. Several proposals have been made for the tolerable temperature values for 600V vinyl-insulated wire and

*q ~ r

600V rubber-insulated wire, used commonly for low-voltage wiring. However, 60°C for continuous use, 100°C for short-time use, and 150°C for a short-circuit should be acceptable levels.

* q Indoor wiring regulations (JEAC8001)

w Japan Electrical Manufactures’ Association, Wire Overcurrent Investigation Committee “ Various characteristics in respect to overcurrent on 600V vinyl wire and 600V rubber-insulated wire” (Institute of Electrical Engineers of Japan Journal Edition 74-791)

e AIEE Transaction RW Jones, JA Scott “Short time current ratings for aircraft wire and cable”

r Institute of Electrical Engineers of Japan, Electric Standards Investigating Committee Standards “Tolerable current for 2-cotton insulated wire, 600V rubber- insulated wire, and 600V vinyl-insulated wire” (JEC-135)

In the continuous and overload ranges, the wire conductor temperature are determined by the heat dissipation. Thus, the wire tolerable current cannot be easily calculated like the short-circuit range. Regarding the use of 600V vinyl-insulated wires and 600V rubber-insulated wires for continuous use, Table 4. 4 shows the wire’s tolerable current set forth in the Electrical Installations Technical Standards Interpretation 172 in which the tolerable temperature of the conductor is 60°C (when the ambient temperature is 30°C, the conductor’s temperature rise value is 30°C). When the conductor’s tolerable temperature is higher than the vinyl wire, such as with a 600V 2-type vinyl- insulated wire (conductor tolerable temperature 75°C) and polyethylene-insulated wire (conductor tolerable temperature 75°C), and cross-linked polyethylene-insulated wire (conductor tolerable temperature 90°C), etc., the values given in Table 4. 5 are compensated by multiplying with the values given in Table 4. 4. Furthermore, with wires laid in a conduit (metal or insulated pipe) are insulated, so the heat dissipation drops and the tolerable current drops. In this case, the above value is multiplied with the coefficient given in Table 4. 6.

Thus, the rated current of MCCB that is supposed to protect these wires must be smaller than the tolerable wire current determined by the above method.

Table 4. 4 Insulation wire’s tolerable current

Table 4. 5 Tolerable current compensation coefficient

Table 4. 6 Compensation coefficient according to conduit

Number of wires in same conduit	Current compensation coefficient
3 or less	0.70
4	0.63
5 or 6	0.56
7 to 15	0.49
16 to 40	0.43
41 to 60	0.39
61 or more	0.34

4. 4. 3 Short-time use range (overload range)

For the actual time of the short-time range where the conductor tolerable temperature is tolerated to 100°C (for vinyl or rubber-insulated wire), *q above suggested several hours and *e suggested 20s or more. However, it can be said that it is about the same as MCCB long-time delay tripping time.

Fig. 4. 16 shows the current time characteristics for a 600V vinyl-insulated wire having a wire ambient temperature of 30°C, which starts with a no-load state, and which has a conductor temperature of 100°C. Fig. 4. 19 to Fig. 4. 22 show the coordination of these current time characteristics and MCCB operating characteristics curve (maximum tripping characteristics curve for each rated current). The figure shows when current time characteristics of the wire is higher than that of MCCB, the wire is protected.

Since Fig. 4. 19 to Fig. 4. 22 show the insulated case, the allowance within the short time range may be too much for wires placed in a conduit. However, the wire current time characteristics curve shown in Fig. 4. 17 obtained using the previous Table 4. 6 compensation coefficient is compared with MCCB.

When studying this wire and MCCB, MCCB operating characteristics curve use the reference ambient temperature 40°C and the wire’s current time characteristics use the ambient temperature 30°C. Normally, MCCB is installed in a panel to protect the wires outside of the panel, so there is no contradiction in comparing in this state.

Fig. 4. 18 shows the relation of the wires that can be protected and MCCB rated current, as seen with Fig. 4. 19 to Fig. 4. 22.

Fig 4.18 Coordination of 600V vinyl-insulated wire and MCCB