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With the earth is shows it will actually work. With ring finals it shows it is a ring. With a radial with no continuity it simply will not work. In real terms we are looking at the reading and using our knowledge gained over the years which tells us what to expect. When I was doing my C&G2391 I failed the installation on the continuity test as the impedance was too high for the distance involved, there was a series of resistors in the test rig to emulate the results expected in a house, I found the resistors and reported them as a serious fire risk, which they were. However the reading was well within spec.Hello guys
Can some one pleas help me with a simple understanding of the following
1) what is continuity testing in an installation
In the main cable does not leak, most low insulation resistances are caused by poor cable joints, be it a socket or a junction box. However there are items which will give false readings. Neons, USB sockets, filtered sockets can all cause the reading to be lower than normal. But again walking around the installation first should make one aware of items which may need isolating to get reading required. But aim is to locate sockets in wet walls and the like.2) insulation resistance testing
In this country we do have single pole switching, polarised sockets, and fuse only in the line of a plug, the latter makes it very important that line and neutral are not swapped even if they are both classed as live.3) polarity testing
The range does vary, not got latest version of BS7671 so not going to quote values in case they have changed again. The loop impedance and prospective short circuit current are related with ohms law, many meters will flick between both reading with a flick of the switch, some auto read line - earth with loop impedance and line - neutral with pscc. There are a number of reasons to measure loop impedance.(I have a fair idea of what they are but look for a more understanding of each)
A) why are they carried out
B) what is the permissible range in testing.
And do you have to work out the diversity factor for every hob or oven even if it falls with in the 32amp reading
Ie an oven at is on a 32 but rated 2.9kw I worked it out and with out using the diversity Mehod it in 32amp use the diversity method and it's even below.
And the hob is 7200w
Lastly any good books out there with simple explanations
Thank you all I know it's a Sunday but if one has a spare time to answer thanks a lot
1) To ensure a MCB will trip with the magnetic part, with with a B type is 3 to 5 times current needed to trip the thermal part of the device. We now also give a 5% extra to be on the safe side. So a B32 will be 32 x 5 + (32 x 5 x 5%) amps, that's 168 amp pscc or 230/168 = 1.37 Ω.
2) To ensure the MCB will not be overloaded if there is a short circuit, often they are rated 10k amps although some RCBO versions can be lower often 7k amps, it is rare to find after the 100A incoming fuse that the pscc is too high, and even if it is we also work with let through current of the fuse, can't remember off hand what a 100A fuse let through amps is, but it is less than 7k.
3) To ensure volt drop is not exceeded, likely this is lower than what is required to trip the MCB, with a B32 MCB with incoming loop impedance of 0.35 Ω likely the limit on a ring final at centre of the ring is 0.97 Ω. However since the incoming impedance can change, and even if over 0.97 Ω in real terms volt drop is unlikely to be exceeded then not that important, since this would be the line - neutral reading which will likely be better than the line - earth reading with a B type the volt drop limit is very close to the magnetic trip limit. One should always read line - earth and line - neutral and go by the worst reading, however in real terms since one is taken in ohms and other in amps we don't really compare.
As to diversity some common sense is needed. With for example an induction hob, within a few minutes of switch on, the mark/space ratio will reduce the average current by quite a large factor, however with a resistive hob the time before the simmerstat starts to switch is a lot longer, also with the induction hob very little energy is wasted but with resistive likely 40% is wasted. If my 40% is correct then a induction hob with a pan of food on it will take 40% less power than the resistive hob with same pan. However the induction hob is likely rated 3.7 kW but the resistive rated at 1.2 kW. So in theory we could allow more diversity with an induction hob than with a resistive hob.
However in a commercial setting food can be rapidly moved on and off the hobs, and although the auto switch off once pan is removed will reduce the power, a cook swapping pans around could cause the hob to give out near the maximum of 1.85 kW there is a maximum time on boost so not full 3.7 kW. So in a commercial setting the cooker could have a higher current draw. But most cookers actually state not for commercial use, so that should not be a problem.
So we look at worst case scenario, what happens if you miss calculate the diversity? The MCB trips. So in real terms no harm done. So my stand alone cooker with everything on could in theory draw over 60 amp, however the manufacturer recommends a B32 MCB, as it would be very hard to draw 60A for more than a couple of minutes before some of the heating units start to cycle.
Having said that there are exceptions, you can get range cookers that only require a 16A supply. You never turn the hot plate off, you just put down the cover and conserve the heat, now these cookers could draw 16A for quite an extended time. So you do need some common sense.
I was told how fitting induction hobs in a commercial kitchen kept the electric bill the same and reduced gas bill. What power was used by the induction hob was matched by the reduction in power used by the air conditioning units. Again a lot depends on the building, and if effective flues can take away the heat which escapes with gas cooking. Gas cooking is very wasteful lucky gas is also very cheap. It is really hard to have a one rule fits all. At some point the designer has to make the executive decision, and stop hiding behind regulations and guides.