Changing mcb type b to rcbo type c

[daveyboy]

Hi guys i'm just finishing my level 2 2330 so please bear with me.

In my house I have a proteus consumer unit with all mcbs and no rcd protection. I was thinking of fitting all individual rcbo's on the power circuits. The current mcbs are type b and I have found some type c rcbos.

Now I know I can change them for a type C as long as it complies with the disconnection times can anyone please give a bit of advice.

Thanks:spam

 

[Andy™]

You say you know you can change it if it complies with disconnection times.... so what advice are you after?

 

[M107]

page 250 & page 49 of the brg are all you need.

 

[green-hornet]

Just remember the maximum permitted Zs for type c are a lot lower than type B

For instance 32 amp type B maximum zs = 1.15, 32 amp type c maximum zs =0.57.

 

[Barx]

Lookup maximum permissible Zs values for the values of rcbos your going to use. Use a Earth fault loop impedence tester to test each circuit and check results against maximums looked up earlier. If yours are lower that MAX. then will be ok.

Else change board for one that you can get type B's for.

 

[daveyboy]

Thanks guys thats exactly what I needed to hear.

 

[M107]

But then the question has to be asked, does the Zs matter seen as an rcbo will be in use

 

[Evans Electric]

But then the question has to be asked, does the Zs matter seen as an rcbo will be in use

An interesting statement Mr M107 I await other posts.

Deke

 

[Badger]

well id still have said yes in that circumstance as the cable could give l-n fault and cause overload or short ciurcuit

 

[Andy™]

Surely Zs isnt as important, since its an RCBO. main thing to check would be L-N loop, to make sure it will trip incase of short circuit between live and neutral

 

[Gforce]

On site guide page 21 3.6.1 rcds are required when earth fault loop impedance is too high to provide the desired disconnection time. Though it does give an example of a tt system would it not be ok in this case as an rcbo is essentially an rcd.

I dont know im just a novice with a good book LOL

 

[green-hornet]

Zs is just as important for a rccbo just as a mcb, if you look at the tables you will see the column Circuit-breakers to BS3871 or BS EN 60898 or RCBO's to BS EN 61009.

They are the maximum permitted Zs readings to ensure under fault conditions the device will disconnect within the time specified.

 

[Crock]

If I change 3 MCBs to RCBOs in a commercial premises do i need to test and issue a certificate?

 

[ADS]

well id still have said yes in that circumstance as the cable could give l-n fault and cause overload or short ciurcuit

But that would have nothing to do with the Zs - so once again, is it important??

 

[ianmacd]

Yes.

 

[gselectrical]

Could you not say that the maximum Zs is 1667 if 0.57 (32a Type c) could not be achieved?

 

[ianmacd]

My Yes was in responce to this:

If I change 3 MCBs to RCBOs in a commercial premises do i need to test and issue a certificate?

 

[steptoe]

please remember gentlemen,

99% of RCBOs in use in the UK are only single pole,

99% of RCDs are double pole

TN values apply to SP RCBOs exactly the same as SP MCBs

only if your supply is TT and then you must use DP RCBOs (or equivalent device) can this argument about max Zs values be applied.

 

[gselectrical]

Appreciate that Steptoe, Will an RCBO not operate the same way as an RCD regardless of whether it is single pole or double pole and therefore the 1667 value be used?? Please correct me because im probably wrong. J

 

[steptoe]

it wont/cant disconnect the Neutral, therefore adjacent faults appear across all circuits on the same N bar.

Im not big up on BRC, but Im sure in there somewhere it says something about disconnection of all live conductors on TT. this is generally not considered necessary on TN systems.