broadgage:

Another point to consider is that an earth fault on a system supplied by a generator is often much less dangerous than a similar fault on a system supplied by public mains.

Consider the following illustrative examples.


1) An installation connected to 240 volt public mains. Resistance of earth conductor is 0.12 ohms and resistance of phase conductor is also 0.12 ohms. Resistance of neutral conductor is 0.12 ohms. Full load current is 50 amps, with a full load voltage drop of 12 volts.

An earth fault occurs and with a total earth loop impedance of 0.24 ohms the fault current will be 1000 amps. Whilst the fault persists there will be a voltage drop of 120 volts in the phase conductor and a voltage rise of 120 volts on the earth conductor. Any person in contact with a class one appliance and with true earth will get a shock a 120 volts. Dangerous, hence the need for prompt disconnection of such faults.


2) Now consider the same installation but supplied from a standby generator instead of public mains. The generator has a full load current of 100 amps and a maximum short circuit current of 250 amps.

An earth to phase fault occurs as previously.

With the fault current limited by the design of the generator, the voltage rise of the earth conductor and of class one appliances connected thereto is now only 30 volts rather than 120 volts. Very low risk, even if the fault persists for longer than would be acceptable under normal conditions with the mains supply.


RCD protection of general use sub circuits will still operate as before and gives extra protection.

Think twice about sensitive earth fault protection on life safety circuits. If the building is on fire, do you really want the fire pump to trip out due to 100ma of earth leakage ?

 

Exactly what I was thinking, the generator will output a lower circuit voltage. However, because the MCB will take longer to clear won't the field or excitation collapse before the MCB opens? I want to avoid this possibility, particularly in a safety supply.

broadgage:

Another point to consider is that an earth fault on a system supplied by a generator is often much less dangerous than a similar fault on a system supplied by public mains.

Consider the following illustrative examples.


1) An installation connected to 240 volt public mains. Resistance of earth conductor is 0.12 ohms and resistance of phase conductor is also 0.12 ohms. Resistance of neutral conductor is 0.12 ohms. Full load current is 50 amps, with a full load voltage drop of 12 volts.

An earth fault occurs and with a total earth loop impedance of 0.24 ohms the fault current will be 1000 amps. Whilst the fault persists there will be a voltage drop of 120 volts in the phase conductor and a voltage rise of 120 volts on the earth conductor. Any person in contact with a class one appliance and with true earth will get a shock a 120 volts. Dangerous, hence the need for prompt disconnection of such faults.


2) Now consider the same installation but supplied from a standby generator instead of public mains. The generator has a full load current of 100 amps and a maximum short circuit current of 250 amps.

An earth to phase fault occurs as previously.

With the fault current limited by the design of the generator, the voltage rise of the earth conductor and of class one appliances connected thereto is now only 30 volts rather than 120 volts. Very low risk, even if the fault persists for longer than would be acceptable under normal conditions with the mains supply.


RCD protection of general use sub circuits will still operate as before and gives extra protection.

Think twice about sensitive earth fault protection on life safety circuits. If the building is on fire, do you really want the fire pump to trip out due to 100ma of earth leakage ?

 

Exactly what I was thinking, the generator will output a lower circuit voltage. However, because the MCB will take longer to clear won't the field or excitation collapse before the MCB opens? I want to avoid this possibility, particularly in a safety supply.