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  1. happyhippydad

    Achieving selectivity from 30mA?

    My customer has some garden pole lights up his driveway, they occasionally trip one of the RCD's in his dual RCD baord (once every 1-2 years). I will fix the problem but they are in such a poor position that I feel it may happen again in a year or 2. They are simply plugged in to a socket in...
  2. C

    Electrical Discrimination Issues

    I have an electrical installation in a garage. The supply from the house to the garage is protected by a 80 Amp 100mA RCD. When I use certain tools in the garage, mainly an orbital sander and a finger grinder the main RCD in the house trips and plunges the place into darkness, seemingly by...
  3. N

    No Discrimination

    Evening guys, Query with discrimination. Main DB with 30m/A RCD feeding another sub board with 30m\A RCD. The RCD in the main DB is tripping out before the sub board RCD. Am I able to change the Main DB RCD to a type S? (All that Main RCD is feeding is two sub boards, twin and earth fed in an...
  4. B

    no discrimination in sockets & lights

    I have came across a problem where the sockets and lights are run on a same circuit and it's done in a way where the person have put probably junction boxes in the walls which are not accessible and only 1 cable is coming to sockets , light switches and lights. I can't put fcu to discriminate as...
  5. N

    Instantaneous faults and discrimination

    Hi, I am currently working on a project where the client has asked us to complete a discrimination study. The primary area of concern involves an upstream miniature circuit breaker (25A type C) and a downstream 10A gG fuse. The type C circuit breaker time current characteristics within BS...
  6. B

    Rcd discrimination on circuit

    Hi, I’m installing some caravan boxes with 32A socket which is protected by 32A 30ma RCBO inside the box and then plug and play 32A socket inside the caravan to consumer unit which has main switch and RCD circuits can some on explain the discrimination please?
  7. Ian1981

    Discrimination charts for square D/Schneider

    Has anyone got any decent discrimination charts for square D/ Schneider mcbs? What I want to know is at what point might I achieve discrimination of some description between a 63 amp type C upstream device and a 32 amp type B downstream device. Zs for the sockets on the type B was 0.53 ohms...
  8. Michaelwgroves

    Using Type D MCB for discrimination

    I've just been on phone with Hager designing domestic distribution board design. Engineer advised using Type D 63A in distribution board supplying CU to give better discrimination. Looking at tables anything over 10A on CU might also trip Type D, but it does give a better chance. So I need to...
  9. Soulsurfer

    MCB discrimination ...

    Hey I had a callout referred by a lad working for me yesterday where one of the old dimplex CXL I think it is storage heater with an on peak convector heater built in had a fault and took out the property supply. He had attended the call and said that the B16 Hager at the board had not tripped...
  10. C

    Garage Board - achieving discrimination?

    Hi all, New to the forum so thanks in advance for your help. I've just fitted a garage board details below: Main Consumer Unit is Dual RCD - whoever did the re-wire had already fitted 6mm t&e direct from cu into an external jb. I've connected the t&e to swa (house end) and at the garage end...
  11. joebell25

    Discrimination headache!!!

    Thought I'd try out this ElectricalOM software for a new installation I'm designing. However come up with abit of an issue with the discrimination on 2 of the devices. Single phase sub feed from DB2 (60898 type c 63A 10kA) to DB3, Final circuit from DB3 (61009 30mA 32A type c 10kA RCBO) Time v...
  12. T

    3 phase distribution

    Hi all, Just an enquiry.. Our company have recently moved factory's. I am talking a very small unit. At the moment there is one TPN distribution board which is at the top end of the factory.. Is there any problem running a 3 phase supply off a 63amp mcb which will feed the new d/b just to keep...
  13. P

    Problems about IDMT relay

    Today i found that there are two questions about IDMT relay which i dont know how to answer. 1. What will haggen if electrician forget to insert the "plug" in the IDMT protection relay? 2. Discrimination is important in electric circuit protection. Please tell me how to apply the plug settings...
  14. mattg4321

    Standby generator wiring

    Hi guys, after some advice for a job currently ongoing. I'm fairly inexperienced when it comes to generators, trying to decide whether or not to take this on. We are currently wiring a domestic new build (mostly!) property (including 4 separate buildings, all with lightings/small power). 90%...
  15. J

    MCCB incomers on paneboards

    Hi I have a question which I am sure can be answered by the chaps on here in seconds. Why do panelboards often have MCCB's as incoming devices over isolating switches? I understand this can be useful if the PFC is high at the board and an incomer like this can protect outgoing devices. Is this...
  16. F

    Amtech and Fault Current and Discrimination

    Hi All, Having done my model for Amtech LV. I have noted that when Descriminating between two breakers the Pscc is taken at the load end of the circuit when analyazing discrimination between breakers. Example being:- The discrimination limit between upstream device Q1 and downstream device...
  17. M

    Cable size for single phase motor control panel

    Hi all, Your help would be appreciated. We are mainly domestic sparx so I havn't much experience with cable sizing for motors. On site, we need to supply a motor control panel which operates the sewage pump. There are in fact two pumps but I think they switch between periodically to prolong...
  18. C

    fuses/protective devices

    can anyone explain per-arcing i2t, arcing i2t, total let through energy. in relation to fuses/protective devices
  19. kingeri

    Domestic Eh? RCD protecting tails

    Some may remember that I've got an upcoming job to move 87 CUs about a meter in council sheltered accommodation. Just found out that they also want all the installations protecting with RCDs. I found this odd cos as far as i remember they all had split load boards with dual RCDs. I queried...
  20. kingeri

    Discrimination between BS1362 fuses

    Fused spur off ring with 13a fuse supplying 4x 3-gang fused socket outlets for a small computer area. Loads not going to be massive. Should the 4x 13a fuses in the 3-gang sockets be changed for a lower rating, e.g. 10a, to allow for discrimination between sockets outlets and the fuse in the...
  21. S

    Discrimination on upstairs board

    Saw an installation today where an upstairs consumer unit is run off a 40A MCB in the main board. Two problems 1) Both boards are protected by 30 ma RCD so no discrimination. 2) There's a 40 A MCB in the second board as a dedicated shower feed - the installers have put a 10.8 kw shower on...
  22. M

    3 Farm buildings

    Hi iv been asked to wire 3 farm buildings. The sub mains have to be RCD protected, Do i 300ma Main swt first ccu rcbo's 100 Ma second and use rcbo's and 30ma main swt and mcb's ccu? Any suggestions would be helpful iv red the green book and me head is hurting lol i could use a 100 ma upfront...
  23. S

    Fused Spur Query

    I need to spur off a ring main to two locations a Boiler Control and Downflow heater. Is there anything wrong with installing one 13A fused spur unit adjacent to a socket on the ring main and running both cables out the back of it to seperate fused spur units in the appropriate locations?
  24. F

    time delay rcds

    were would you use a time delay rcd and what are the benefits of doing so is it just in tt installations
  25. N

    30m/A protection on outside sockets

    As I'm aware all outside sockets must have at least 30m/A RCD protection . However.... The DB this socket is fed from has a 100m/A main RCD (the socket is off a 10A MCB). So it doesn't comply. But....this DB is a submains and is fed from another DB which DOES have a 30m/A main RCD. So does...
  26. C

    New supply tails in trunking or SWA?

    Hi, doing a garage conversion into habitable room and will have to move the CU. Currently it's at high level above the entry where the meter tails come through the wall. The CU needs to be moved to make way for a doorway and will be fitted approx 6M from existing position on the other side of...
  27. W

    Farm Installation help

    Hi all Went to look at a job today on a farm , 3 phase board with 75mm supply on a 100 amp 100 ma rcd mainswitch, with a submains out to the house , which is also protected by another 100 ma rcd, problem is every time theirs a trip in the house it trips at the main distribution board also which...
  28. La Poste

    RCD's in series

    I was having a chat with my electrical boss today about this and that and we discussed wiring a radial in a garden supplying various things like a greenhouse and a garden shed. He was of the opinion that things like greenhouses should be TT'd with an earth spike and RCD protected. The thing is...
  29. L

    Discrimination of fuses?

    Hiya, Just wondering if any of you would be able to help me understand the I2t curves for discrimination between protective devices. (I have read that if the major fuse is twice the rating of the minor fuse then discrimination is achieved, but I am trying to work it from the I2t curves) I have...
  30. Y

    Linking two fused connection units from a junction box

    Advice please .Is it ok to put a junction box into a ring main then from that put a fcu with a 13amp fuse for a cooker hood, then from that fcu put another fcu with a 5amp fuse for 2 under cabinet strip lights using the correct size wires etc ?
  31. V

    high readings on RCD test

    just a quick one lads, I was sent to a job to change a trip in a pirilli box that was covering the cable going into the house. Then I changed the trip in the house that was covering the house its self. My foreman give me 100Ma time delay for outside and a 30Ma for inside, I thought that we had...
  32. M


    I have a SWA cable running from my cu in the house to a cu unit in the garage. Does this cable have to be RCD protected?
  33. T

    another old fuse box

    hi all, here's another one i saw today which i wondered about. my mate was proudly showing me his new bathroom and i thought i'd have a nose at the electrical arrangements. he too has another old 1361 fuse box, with a new 13A 30mA RCD FCU installed on the wall above said fuse box providing the...
  34. L

    Sockets under Sink ?

    Most new kitchens you seem to have no option but to install sockets (for wash machine/dish washer etc) under the sink in the base unit. I do not believe there is any reg's restricting this but some kitchen fitters i come across still seem to think it is taboo (though they do it anyway). Am i...
  35. stidge

    RCD to RCd

    Running a feed to garage from dual RCD CU.Would like to fit RCD CU in garage but have heard that this might cause nuisance tripping( RCDs acting against each other)???????? Have seen this before on various jobs ie RCD controlled radial to RCD socket !!! Would like your opinions/answers on this...
  36. C

    60A Main fuse & NIC assesment?

    I was in my brothers house yesterday as I am possibly going to do a job there for one of my NIC assesments, and he has a 100A main switch, but the main fuse is only 60A (in a 100A holder). Ill check the size of the meter tails next week, but if they are suffecient, should we ask for the fuse to...
  37. K

    Enough Fuse Discrimination?

    Evening all, I am a newbie to this site and also to electrical work. I am currently a young road lighting technician at the local council and am studying for a HNC in electrical engineering, a G39 and a Diploma in road lighting standards. I have been in contact with a consultant and he has...
  38. S

    RCD discrimination

    Hello everybody. I would like to understand how discrimination works for rcd's if say for say a garage supply you would put in a 100mA rcd, while the main board would be protected by a regular 30mA. So this ensures that a fault in the garage will not disturb the point of origin but I don't...
  39. R

    Domestic Outside Socket

    Hi there, This is my first post hope you can help. I have been asked to fit an outside socket so was going to split the kitchen ring main and loop to the outside socket then back to the kitchen ring. The CU is single RCD portected for all circuits and the outside socket is IP56 with RCD...
  40. K

    RCD protection query

    I am adding a switch (the wiring which will be chased less than 50mm deep into the wall) to a lighting circuit in a house where all the circuits are protected by 1 RCD, obviously this does not provide any discrimination between circuits. In order to provide RCD protection for the lighting...
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