understanding the importance of zs

[chrisreid186]

hi guys just something that's been bugging me, as i understand it a MCB trips on overload (to much current) and a RCD trips on a imbalance of amps through live / neutral , its important to have a low resistance fault path for the protection devices to operate promptly but wouldn't a mcb trip regardless of the fault loop?

am i right in saying the mcb will trip just above its rating but this would take a long time so the low resistance fault loop is there to encourage more amps to flow therefore blowing the quickly

 

[derek preece]

hi guys just something that's been bugging me, as i understand it a MCB trips on overload (to much current) and a RCD trips on a imbalance of amps through live / neutral , its important to have a low resistance fault path for the protection devices to operate promptly but wouldn't a mcb trip regardless of the fault loop?

No, if the resistance is too high not enough current will flow to allow the CB to trip in the required timescale; the graphs for trip times are in Apppendix 3 of the regs book.

am i right in saying the mcb will trip just above its rating but this would take a long time so the low resistance fault loop is there to encourage more amps to flow therefore blowing the quickly

Basically yes, cables also need to be able to handle the heat generated by an Earth Fault so it will not create too much resistance to the flow of current (or explode). That is what I've managed to gather so far anyway still learning myself

 

[chrisreid186]

'No, if the resistance is too high not enough current will flow to allow the CB to trip'
but If i plugged say a 32amp electric hob into a 6 amp mcb and disconnected the earth it should still trip due to the overload

 

[Sparky Ninja]

Zs is required to make sure the device operates within the required disconnection times.
When cables are selected the disconnection times are used within the adiabatic equation to obtain the minimum required conductor size.
Now if we can't achieve these disconnection times then as you have pointed, the conductor will be subject to an increase in heat from the higher current, this is likely to deteriorate the cable.
Devices have different maximum Zs values by design, and devices that are particularly selected for higher inrush currents (types C & D etc) you will notice allow for a much lower Zs maximum.

 

[johnboy6083]

zs is earth fault loop impedance. in basic terms, the resistance of the path the current will take in the event of a earth fault.

The lower the ZS value (the resistance), then the more current that will flow in a specified time, and the quicker the protective device will open the circuit.

It is imprortant to make sure that the breaking capacity of your protective device is more than the value of the fault current. Otherwise, the protective device will not be able to interupt the current, and this can cause fires, and problems further up the distribution system.

Breaking capacity is usuall written on the side of fuses/mcbs, and on the front of MCCB's/ACB's. on mcb's it is usually written as a number in a black square, and that number is the value of the breaking capacity in Kilo Amps, ie 10 would be 10,000 amps.

 

[chrisreid186]

cheers for quick advice guys seems a little odd that you have to make the fault more dangerous (more amps) to make it safe quicker, but then its not really more dangerous because the duration is less

 

[derek preece]

'No, if the resistance is too high not enough current will flow to allow the CB to trip'
but If i plugged say a 32amp electric hob into a 6 amp mcb and disconnected the earth it should still trip due to the overload

Yes the breaker will still trip acording to the trip times in the regs book due to overload.

 

[morph]

i think you may be confussing overload with fault protection..... overload occures without the earth needing to be atached, fault protection relies on the earth for reduced impedance to alow high current flow for short periods however fault protection can still be achieved without the earth as in a dead short but the disconections times would not be met....and the fault current would be very high ie PFC ...making sense now , good stuff aint it .

hi guys just something that's been bugging me, as i understand it a MCB trips on overload (to much current) and a RCD trips on a imbalance of amps through live / neutral , its important to have a low resistance fault path for the protection devices to operate promptly but wouldn't a mcb trip regardless of the fault loop?

am i right in saying the mcb will trip just above its rating but this would take a long time so the low resistance fault loop is there to encourage more amps to flow therefore blowing the quickly

 

[Mark Wright]

Everywhere MCBs are Defined as OverCurrent/Overload and Fault Current/Short Circuit Protection and not Earth Fault.

Often people describe MCBs as not offering Earth Fault protection yet we're checking our Ze/Zs to make sure the MCB will trip under Earth Fault conditions.

 

[telectrix]

Everywhere MCBs are Defined as OverCurrent/Overload and Fault Current/Short Circuit Protection and not Earth Fault.

Often people describe MCBs as not offering Earth Fault protection yet we're checking our Ze/Zs to make sure the MCB will trip under Earth Fault conditions.

true, but the use of Zs figures in accordance with the regs. is to ensure disconnection within a certain time in the case of a fault of negligible impedance (short circiuit) between Live parts and exposed conductive parts. RCDs are to protect againsy human contact with live parts. the fact that they will trip in the case of a L-E or N-E fault is a bonus, but not to be relied upon.

 

[Mark Wright]

Yes Thanks just a bit frustrating that i've only just realised in these conditions a MCB will Trip under Earth Fault after reading numerous text and bothering people with questions on the subject and thinking this was not the case.

 

[Midwest]

How do you find these old threads @Mark Wright

 

[Mark Wright]

How do you find these old threads @Mark Wright

basically searching for what OP was asking.

 

[static zap]

@Midwest

searching for what OP was asking.

That will be my fault then - Pressing Dan for a better search