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Disconnection times on a system if a stand by generator is used

Anthony Smith

On a fully designed power distribution system, clearly the fault levels, earth loop impedances and ultimately fuse disconnection times are determined by the system parameters Transformer %Z etc one or two in parallel for min and max fault levels levels. What needs to be considered when bringing on a stand-by generator, clearly all Fault levels, Earth loop impedances and consequently fuse disconnection times would change because of the Generator parameters. How would you design in these new parameters, bearing in mind that disconnection times on a large system may no longer be achievable 

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    It might, exceptionally, be considered acceptable to use engineering judgement to permit of a longer disconnection time, under generator power, than would ordinarily be allowed.

    Consider the following. Q why do we require that earth faults be disconnected in some specified and short time ?  A to reduce the danger from touch voltages during the fault. During an earth fault on a standard mains supply, it is likely that the supply voltage will be maintained at close to the nominal voltage of 240 volts.

    Presuming equal voltage drops in the line conductor and in the CPC, That will give about 120 volts on exposed metal work during the fault. Dangerous, hence the requirement to disconnect such faults promptly.

    Now consider an earth fault on generator power. During the fault, the generator output voltage will NOT remain even roughly constant but will decay to a very low figure. The touch voltage will be half of this much reduced voltage. 

    This only applies to circuits such as sub mains that are large in relation to the generator capacity. On small final circuits the generator should operate the OCPD device promptly.

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    It might, exceptionally, be considered acceptable to use engineering judgement to permit of a longer disconnection time, under generator power, than would ordinarily be allowed.

    Consider the following. Q why do we require that earth faults be disconnected in some specified and short time ?  A to reduce the danger from touch voltages during the fault. During an earth fault on a standard mains supply, it is likely that the supply voltage will be maintained at close to the nominal voltage of 240 volts.

    Presuming equal voltage drops in the line conductor and in the CPC, That will give about 120 volts on exposed metal work during the fault. Dangerous, hence the requirement to disconnect such faults promptly.

    Now consider an earth fault on generator power. During the fault, the generator output voltage will NOT remain even roughly constant but will decay to a very low figure. The touch voltage will be half of this much reduced voltage. 

    This only applies to circuits such as sub mains that are large in relation to the generator capacity. On small final circuits the generator should operate the OCPD device promptly.

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