147ohms ZS, 30mA RCBO 16A type D. Will it trip?
- Thread starter Electrox
- Start date
Sponsored Links
With a Live to Earth short I = 230/147 = 1.56 Amps = 1560 mA in Live from RCBO, virtually zero in Neutral from RCBO
Un-balance at RCBO is therefor 1560 mA
1560 mA is much greater than 30mA so RCB part will trip
1.56 Amp is less that 16 so MCB part will not trip.
Hence the RCD part is providing potection for this fault.
A Live to Neutral short ( or excessive loading ) the un-balance is almost zero so the RCD part will not trip. But if the short circuit or overload current is greater that 16 Amps then the MCB part will eventually trip. How fast depends on the current.
- Joined
- 27 Jan 2008
- Messages
- 23,668
- Reaction score
- 2,666
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
Simple answer is yes it will trip.
As already pointed out the RCBO is made up of three parts.
1. 30ma imbalance between line and neutral will trip it.
2. 16A for an extended time will trip the thermal part.
3. 320A will trip the magnetic part within milliseconds.
The trip protects for both line - earth faults and line - neutral faults. As a result the line - neutral impedance is also important. This is normally given as fault current and as long is it is greater then 320A then it will trip within milliseconds. However between 16A and 320A the trip time may be a lot longer and it is up to the installer to determine if this is likely to be a problem. Where there is a secondary method of tripping for example a 13A fuse or motor contactor it may be considered that the risk is minimal.
The old MCB Loadmaster Dorman Smith did not have a magnet part and there did not seem to be too many problems when using them. It may be with a RCD built into the MCB that really the magnetic part is not longer required! However we are not suppose to rely on a RCD although that is what we do with a TT system. The RCD part of the RCBO is the only part we test. The other parts we just accept will work when required.
There is also many types of RCD and we tend to ignore the limitations are it is rare to see anyone do a risk assessment before fitting one.
So why do you ask?
As already pointed out the RCBO is made up of three parts.
1. 30ma imbalance between line and neutral will trip it.
2. 16A for an extended time will trip the thermal part.
3. 320A will trip the magnetic part within milliseconds.
The trip protects for both line - earth faults and line - neutral faults. As a result the line - neutral impedance is also important. This is normally given as fault current and as long is it is greater then 320A then it will trip within milliseconds. However between 16A and 320A the trip time may be a lot longer and it is up to the installer to determine if this is likely to be a problem. Where there is a secondary method of tripping for example a 13A fuse or motor contactor it may be considered that the risk is minimal.
The old MCB Loadmaster Dorman Smith did not have a magnet part and there did not seem to be too many problems when using them. It may be with a RCD built into the MCB that really the magnetic part is not longer required! However we are not suppose to rely on a RCD although that is what we do with a TT system. The RCD part of the RCBO is the only part we test. The other parts we just accept will work when required.
There is also many types of RCD and we tend to ignore the limitations are it is rare to see anyone do a risk assessment before fitting one.
So why do you ask?
I'm not a spark but a couple of questions (youve probably already addressed) are why a D type? are you supplying motors or something?
147ohm is a pass on a TT (unless your boss or competent person scheme says otherwise), but it is on the high side, should you also be reducing that as it will likely rise as the sun comes out more.
147ohm is a pass on a TT (unless your boss or competent person scheme says otherwise), but it is on the high side, should you also be reducing that as it will likely rise as the sun comes out more.
Sponsored Links
If I were a betting man my money would be going on him being at college and trying to get other people to do his work and his thinking for him...
Now for the bonus question, what if the Zs was 72ohms, but the device was a D32 RCBO, Ze is 0.30 and the system is TNCS
(Ok... its a circuit a mile long, and volt drop will be way out of spec, but be arnt worried about that for now)
(Ok... its a circuit a mile long, and volt drop will be way out of spec, but be arnt worried about that for now)
If he goes to college in Leeds, no doubt he's spent most of his afternoons in the palace like we all used to
- Joined
- 1 Jan 2009
- Messages
- 254
- Reaction score
- 2
- Country
If I were a betting man my money would be going on him being at college and trying to get other people to do his work and his thinking for him...
You would be amazed at how many times a person can ask for the same formula on some forums.
S
sjwkmje
If he goes to college in Leeds, no doubt he's spent most of his afternoons in the palace like we all used to
What place is that? I am at Leeds uni and I go to the library (actually its a pub just outside the engineering department!) But I do not expect other people to do my homework
Adam
It's a pub on the calls, down the bottom of end of town out past the bus station and quarry hill flats.
http://wikimapia.org/#lat=53.794737&lon=-1.5345803&z=19&l=0&m=a&v=2
http://wikimapia.org/#lat=53.794737&lon=-1.5345803&z=19&l=0&m=a&v=2
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
