IET logo
 
IET
Decrease font size
Increase font size
Topic Title: HRC Fuse and MCB Discrimination
Topic Summary:
Created On: 30 October 2009 11:14 AM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 30 October 2009 11:14 AM
User is offline View Users Profile Print this message



abrooker

Posts: 49
Joined: 28 July 2005

I am would like some advice as whether discrimination would achieved in the following arrangements.
This is to supply socket or cooker circuits in domestic installation via a main 230V SPN disboard and local consumer units.

Incoming supply is supply is 230VTNC-S, with a measured Ze of 0.21 ohms.
The RCD and cable protection complies with 17th Edition.

Number 1:-
Incoming : 80A HRC BS1361
Disboard: 50A MCB type B
Consumer unit: 32A MCB type B

Number 2:-
Incoming : 80A HRC BS1361
Disboard: 50A MCB type B
Consumer unit: 30A HRC BS1361

Number 3:-
Incoming : 80A HRC BS1361
Disboard: 45A HRC BS1361
Consumer unit: 30A HRC BS1361

Thanks for the help and assistance.
 30 October 2009 11:26 AM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8891
Joined: 03 October 2005

Firstly you have to remember that fuse to circuit-breaker you only have partial discrimination.

Why not obtain the time/current characteristics of the devices you mention and do your own comparison.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 30 October 2009 11:51 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19851
Joined: 23 March 2004

The RCD and cable protection complies with 17th Edition.


Where's the RCD in this arrangement. If close up to the 80A HRC then you can't achieve discrimination under earth fault in any circumstances.

For short Circuit:

It will be almost impossible to get discrimination between the 50A and 32A MCB's on incommer 1.

You almost certainly won't get discrimination between the 50A MCB and the 30A HRC - it may work the other way round

You might just get discrimination between 45A and 30A BS 1361's - depends on the fault level at that point but clearing the 30A fuse will almost certainly stress the 45A fuse intolerably

I guess the key question is why you think you need discrimination at all - this sounds like a domestic (sheltered housing perhaps).

As RnR suggests - obtain the T/C curves and do a basic overlay if you want some certainty (probably that it won't discriminate rather than it will)

Regards

OMS

-------------------------
Failure is always an option
 03 November 2009 11:39 AM
User is offline View Users Profile Print this message



abrooker

Posts: 49
Joined: 28 July 2005

Thanks R&R and OMS for the replys and advice.

I have done a basic overlay study using the T/C curves from BS7671 Appendix 3.

Arrangement Number 2 is definately a no-no for discrimination.

Arrangement Number 1 is suspect due to 3 to 5 times In operating tolereance of type B MCBs.

Arrangement Number 3 "just" discriminates according to the Eaton MEM T/C data for their BS1361 HRCs.
The clearing current of their MEM 30A HRC is about 80A less than the pre arcing of their 45A HRC.
However, agree with OMS that the 45A would probably be stressed intolerably for future service.
Fortunately, the HRCs in this installation are from MEM.

Additional Information
The 30mA DP RCDs in this installation are located in one of two places, either:-
Just after the main SPN disboard at the start of the sub main, to the local consumer unit, in a metalclad enclosure.
Or, as the main incoming control switch for the the local consumer unit.
There is approx 15m of 3 core SWA XPLE cable between the main disboard and the local consumer units.

Regards and thanks.
 03 November 2009 07:52 PM
User is offline View Users Profile Print this message



Kev

Posts: 24
Joined: 27 April 2002


From memory the standard generic ratio between fuses to ensure discrimination is 1.6 : 1.0 .If more accuracy is needed then you should consult the characturistic curves provided by the fuse/cb manufacturers if they aren't printed with BS7671.

Hope this helps..

Kev
 04 November 2009 01:17 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11692
Joined: 13 August 2003

From memory the standard generic ratio between fuses to ensure discrimination is 1.6 : 1.0

I've got 2:1 in my head from somewhere - I think that's for a mixture of any type of fuses though - 1.6:1 is probably right where both fuses are of the same kind.

- Andy.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.