Questions
- Thread starter Jacobssi
- Start date
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H
holmslaw
..
where all metal is bonded together but not to earth, in essence a faraday cage.
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H
holmslaw
..
Protection by electrical separation is sometimes accompanied by earth free eq. bonding, not your average domestic installation by far! You may come across it in test areas in industrial or commercial installs, or in small generator fed supplies (up to about 2400VA iirc).
17th edition states
regulation
531.5.1 where protection is provided by an rcd and disconnection following a first fault is not envisaged, the non-operating current of the device shall be at least equal to the current which circulates on the first fault to earth of negligible impedance affecting a line conductor.
Does this mean that the fault current will be shown in the RCM without disconnecting but when a second fault appears it will be of a equal/greater fault current to the rcd to operate it? So it would take the value of two faults to operate?
regulation
531.5.1 where protection is provided by an rcd and disconnection following a first fault is not envisaged, the non-operating current of the device shall be at least equal to the current which circulates on the first fault to earth of negligible impedance affecting a line conductor.
Does this mean that the fault current will be shown in the RCM without disconnecting but when a second fault appears it will be of a equal/greater fault current to the rcd to operate it? So it would take the value of two faults to operate?
536 Co-ordination of protective devices
a lower breaking capacity is permitted if another device (a back up protection device) having the neccessary breaking capacity is installed on the supply side and the characteristics of the devices are suitably ci
o-ordinated such that the energy let through of upstream device does not exceed that which can be withstood without damage by downstream device.
Is this interpreted as the upstream device is the back up device and the protective normal device the downstream. The let through energy in normal service of the upstream device cannot be bigger than the operating current (without damage)?
a lower breaking capacity is permitted if another device (a back up protection device) having the neccessary breaking capacity is installed on the supply side and the characteristics of the devices are suitably ci
o-ordinated such that the energy let through of upstream device does not exceed that which can be withstood without damage by downstream device.
Is this interpreted as the upstream device is the back up device and the protective normal device the downstream. The let through energy in normal service of the upstream device cannot be bigger than the operating current (without damage)?
IT systems are again, something you'll probably never come across in domestic situations. I have only ever seen one system which I would categorise under this, it was fed from an isolation transformer with the neutral tied to earth via a wire wound resistor and was protected by an RCD (10mA iirc).
It was installed in a test area. The advantage would be that in the event of a fault the current flowing down the CPC is limited by the resistor and the RCD would operate. The resistor must allow the full required current to operate the RCD to flow in a Line to earth fault.
It was installed in a test area. The advantage would be that in the event of a fault the current flowing down the CPC is limited by the resistor and the RCD would operate. The resistor must allow the full required current to operate the RCD to flow in a Line to earth fault.
The energy let through of the upstream device should not exceed the withstand value of the downstream device, these are values which you should be able to get from the manufacturers of the protective device.
These are measured in Amp Squared Seconds (A²s) or I²t which is relative to energy in joules, not the operating value of the protective device.
For example if you have a fault level of 15KA you may install a 10KA MCB (or even possibly a 6KA or less) protected by an upstream device such as a BS88 fuse providing the BS88 fuse has a greater capacity than 15KA and the fuses let through energy does not exceed the withstand energy value of the MCB.
These are measured in Amp Squared Seconds (A²s) or I²t which is relative to energy in joules, not the operating value of the protective device.
For example if you have a fault level of 15KA you may install a 10KA MCB (or even possibly a 6KA or less) protected by an upstream device such as a BS88 fuse providing the BS88 fuse has a greater capacity than 15KA and the fuses let through energy does not exceed the withstand energy value of the MCB.
No - the fuses are upstream of the MCBs, and their let-through energy has to be no greater than the MCBs can tolerate.
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