Is type 3 and Type C the same

ericmark · Post by **ericmark** » Wed Mar 23, 2022 1:28 pm

The compressor would likely have an overload like this

set to motor amps, so the one in the consumer unit is to protect the cable, and not worried about what the existing MCB is, what is of interest is cable size and type and length.

glassescase · Post by **glassescase** » Wed Mar 23, 2022 2:36 pm

ericmark wrote: ↑Wed Mar 23, 2022 1:28 pm The compressor would likely have an overload like this set to motor amps, so the one in the consumer unit is to protect the cable, and not worried about what the existing MCB is, what is of interest is cable size and type and length.

The cable is 6mm and 10 to 15 meters long

glassescase

OnlyMe · Post by **OnlyMe** » Wed Mar 23, 2022 6:29 pm

glassescase wrote: ↑Tue Mar 22, 2022 9:41 pm
OnlyMe wrote: ↑Tue Mar 22, 2022 7:36 pm You need to know what type of supply you have to the house. If you do not know what that means it's probably easier to post a photo of your electrical cut out/main incoming fuse etc and I am sure we can tell you. Or a sticker saying PME connection on the main fuse holder usually gives the game away:-)

The worst case is a TN-S supply with no RCD. Any idea of the length of cable from the CU to the compressor? Is it direct from the main CU or from from a garage CU? These questions and their answers do make a difference.
compressor is served from the consumer unit in the garage, a bit over 10 meters may be around 15 meters. The house consumer unit has a 40 amp cct breaker serving the garage. The incoming main goes to a "grey box" that has series 7, type 2b and BS1361 stamped on it. I say 2b, it is roman numerals like a capitol i but I cant type a proper capitol i. From there to a meter and from the meter to a fuse? The consumer unit in the garage did have a fuse location already marked up as compressor. Could not see a sticker.

glassescase

Tricky as you need to know the distance from the house CU to the garage CU and then add on the compressor cable length. The series 7 Henley cut out is for PME setups so that may be in your favour. I assume it has a nice chunky earth cable coming from the series 7 to your house CU.

ericmark · Post by **ericmark** » Thu Mar 24, 2022 3:13 pm

Measured Ze i.e. at consumer unit not given so I will assume 0.35Ω with a Zs of 0.46Ω that would be around a 15 meter run with 6 mm² your allowed 0.68Ω so should be well within limits.

However it is still guess work until you use a loop impedance meter and take real readings.

glassescase · Post by **glassescase** » Thu Mar 24, 2022 3:21 pm

ericmark wrote: ↑Thu Mar 24, 2022 3:13 pm Measured Ze i.e. at consumer unit not given so I will assume 0.35Ω with a Zs of 0.46Ω that would be around a 15 meter run with 6 mm² your allowed 0.68Ω so should be well within limits.

However it is still guess work until you use a loop impedance meter and take real readings.

unless I can do that with a multi meter doing an ohms test then I dont think I can do that.
A sparky did check my garage consumer unit a few months ago and said it was suitable for my compressor. He was adding a spur in the house for me. Should have done it myself and save £80

glassescase

ericmark · Post by **ericmark** » Fri Mar 25, 2022 10:01 am

No the multi-meter can't measure loop impedance. The electrician often has a case with three meters in it.
1) RCD tester, it measures time as really can't measure 40 mS with a stop watch.
2) The loop impedance tester, often can be set to show as ohms or prospective short circuit current.
3) Combined meter to show low ohms (needs to use at least 200 mA) and insulation tester normally at 500 volt, although most can be switched 250, 500 or 1000 volt.

Some times the meters are combined into one, and seen some which can also test earth rods.

In his kit he should have a way to prove dead, a voltage indicator with no switches so can't be by accident turned off, and a proving unit to show it works, only worked on one job, where this was always done, would have pass removed (i.e. sacked) if found on a job without the tester and proving unit, bit daft as proving unit produced only 500 volt, so would not know if the tester would actually show if 50 volt, so really it was a watch my back exercise.

As to volt meter, ammeter, and ohm meter not so sure really required as the meters already talked about tend to do that, but the clamp-on ammeter is really needed with a scale which can measure mA and DC as well as AC, this is to check on back ground earth leakage and DC injection from items like solar panels and EV charging points. The clamp-on also is a multi-meter in most cases so will measure volts, ohms and often frequency as well.

However since homes are wired before we have the supply from the DNO it is permitted to use inquiry, so I can ask the DNO what the earth loop impedance will be, normal answer is better than 0.35 ohms. And I can use the low ohm meter to measure the resistance of the cables, the add measured resistance to declared impedance to get the figures.

Note with AC called impedance and DC resistance, in the main it is the same.

The low ohm meter as well as using 200 mA measures down to 1/100th of an ohm, most multi-meter will not measure such a small value, I suppose one could work it out using a fixed load and voltage at origin and equipment, so if 232 volt at first CU and 228 volt at the compressor with a 30 amp load then 4/30 = 0.13 ohm but the problem is voltage tends to vary, and you have to consider how accurate so could be really 231.5 and 228.5 or 232.5 and 227.5 so could be anywhere between 0.1 and 0.16 ohms, even if voltage steady, the normal is to used the loop impedance meter three times, and select the mean, well would be better to select mode but does not really work with three readings.

In real terms if well within limits no point taking three readings, it is the same with a lot of calculations which should be done, if one is asked to put a 32 amp supply to a point only 5 meters from the CU you know without measuring it will comply, so one does it, then measures just to be able to enter figures on the minor works certificate. But when looking a a warehouse and a 40 meter run, then one works it out first. Unless you have done it many times before and have a good idea what the results will be.

I have measured and made out so many installation certificates I am sure I could look at most home and fudge up some results without measuring a thing which would be close enough for no one to know I had not measured, until some thing goes wrong, as with the Emma Shaw case. The electricians mate it seems did just that, and so Emma Shaw is no more.

Today will RCD protection the accident would not have happened, as with most accidents it was not one fault, but a list of them, however as long as the circuit is RCD protected then the earth loop impedance being too high is not really a danger.

We are told with a TN supply the RCD is secondary protection, and with a TT supply primary protection, so with this house and a TN supply although the RCBO's (MCB and RCD combined) are type AC and really should be type A, I am not worried, if one is frozen due to DC to also need the RCD to trip at same time is remote, specially when split into 14 circuits. If it was a TT supply then I would spend out and replace some of the type AC for type A.

In other words I have done a risk assessment, maybe not written down as such, but still done it. You need to do the same, the problem is do you have the skill to make that judgement? Fools rush in where angles fear to tread, only you can decide it you have the skill.

I would get a gas safe guy to test my last home, as I did not have the knowledge or test kit, same here with the boiler man, he tests the flue gases etc, and tells me if safe, I am not going to buy the test equipment to use it once every two years or so, better to get some one in.