Does the magnetic part of a MCB matter when using a RCD?

[ericmark]

Does the magnetic part of a MCB matter when using a RCD?

The first thing I look at is the RCBO which come in type B or C so one would consider yes it does.

Although the magnetic part is required where the loop impedance is below the threshold the question is does the loop impedance need to be low enough to trip the magnetic part?

For different systems we see a range of maximum disconnection times from 40 ms, (30 ma RCD) 70 ms,(RCD) 0.2 s,(Table 41.1) 0.4 s, (Table 41.2/3) 5 s (Table 41.4) but these are it would seem for line – earth faults which is covered by the RCD today. At around 10 seconds depending on the type the thermal part of the MCB takes over so only if it needs to trip in less than 10 seconds does one need the magnetic part need to work.

If answer is yes then we need to measure line – neutral loop impedance yet IET forms do not have a provision for this reading so rarely measured. Thoughts please.

 

[Spark123]

It is more to do with the type of load you are applying as opposed to the efli, the efli should be low enough to disconnect in the given time 0.4s for <=32A on a TN system. An example may be using a transformer with a B type where the instantaneous current at startup throws the magnetic part of the RCBO.

A fault between live conductors may be of a lot longer duration than those of phase to earth, usually up to 5 seconds which is quite easily to obtain - volt drop will usually bite before a short between lives is a problem.

 

[sparkiemike]

Does the magnetic part of a MCB matter when using a RCD?...

If answer is yes then we need to measure line – neutral loop impedance yet IET forms do not have a provision for this reading so rarely measured. Thoughts please.

Yes it does matter, your design needs to provide short circuit protection so your line/neutral impedance should not exceed the values given the tables.

I guess with the reduced csa of the cpc we use, the assumption is that the earth fault loop impednace will be higher than the line/neutral impedance, if you are going to use RCDs for fault protection, then you still need to check your results for short circuit protection

 

[ericmark]

Does the magnetic part of a MCB matter when using a RCD?...

If answer is yes then we need to measure line – neutral loop impedance yet IET forms do not have a provision for this reading so rarely measured. Thoughts please.

Yes it does matter, your design needs to provide short circuit protection so your line/neutral impedance should not exceed the values given the tables.

I guess with the reduced csa of the cpc we use, the assumption is that the earth fault loop impednace will be higher than the line/neutral impedance, if you are going to use RCDs for fault protection, then you still need to check your results for short circuit protection

This was also what I considered but after some one questioned it I started to look in the regulations for disconnect times line - neutral as seemed to be line - earth. Clearly where the line - neutral loop impedance is low you will still want the magnetic part to disconnect quickly to avoid damage.

However where the line - neutral impedance has exceeded the value which will give a 0.01 second disconnection time does it really matter if it takes even 20 seconds to disconnect as clearly it will disconnect before the cable overheats.

I have been preaching for a long time on how we should measure the line - neutral impedance and have been saying how important it is as there is a threshold between around 0.01 seconds for anything which can trip the magnetic part which jumps to around 10 seconds once one relies on the thermal part.

But on another forum this was questioned and I started to think. Then I started to look in the regs book. And I can't find any reference to time with line - neutral only with line - earth. So maybe I was wrong?

 

[ericmark]

It is more to do with the type of load you are applying as opposed to the efli, the efli should be low enough to disconnect in the given time 0.4s for <=32A on a TN system. An example may be using a transformer with a B type where the instantaneous current at startup throws the magnetic part of the RCBO.

A fault between live conductors may be of a lot longer duration than those of phase to earth, usually up to 5 seconds which is quite easily to obtain - volt drop will usually bite before a short between lives is a problem.

I don't think I explained the question from your answer. I fully understand inrush what I am asking does in matter if the line - neutral impedance is too high to trip the magnetic part of the trip?

The transformer is a typical supply when the line - earth loop impedance is not low enough to cause the magnetic part of the trip to work. The question on trips being fitted to the 55 - 0 - 55 output from a transformer has been raised many times. Where manufactures have put the trip on the incomer rather than the outgoing side and with a line - line fault it will still function. But with a line - earth fault only half of the winding is being used so it can end up melting the flex rather than opening the trip. Even when on the output a 16 A supply with a B16 MCB will need 80 A to flow at 55 volts this is a loop impedance of 0.69 ohms. Not the 2.9 ohms needs on 230 volt side. Many of the yellow brick transformers don't have an internal impedance of less than 0.69 ohms so could never trip the magnetic part of the trip.

So either all the yellow brick transformers should be banned or having a high loop impedance line - line does not matter as long as there is an RCD. I have still not seen any yellow brick transformers with RCD protection it would be interesting to see if amendment 1 has given them an exemption?

 

[sparkiemike]

I have been preaching for a long time on how we should measure the line - neutral impedance and have been saying how important it is as there is a threshold between around 0.01 seconds for anything which can trip the magnetic part which jumps to around 10 seconds once one relies on the thermal part.

But on another forum this was questioned and I started to think. Then I started to look in the regs book. And I can't find any reference to time with line - neutral only with line - earth. So maybe I was wrong?

Well I suppose you are talking about short circuit current and over current protection, both of which need to be considered in your design

Have you got a copy of the IET "Electrical Installation Design Guide", it covers these two design considerations, and its guidenance is that for simplicity line/neutral impedance should not exceed the values in the tables.

EDIT: It does go to say if your line/neutral impedance exceed the values in the tables, then you need to look at 435.1 and 434.5.2

In terms of risk it would be earth fault current that has a higher priority which I suppose the emphasis on recording it.

EDIT: Also you would only record line/neutral impedance & compare against the tables if that is what the designer intended

 

[bernardgreen]

It appears to me that all these specified figures of current and operating times relate to a fault with a zero ohm resistance.

Is there a "specified" maximum impedence for a short circuit fault. After all the loop impedance for the fault current is not only the resistance of the cables but also includes the resistance / impedance of the fault.

 

[Spark123]

I fully understand inrush what I am asking does in matter if the line - neutral impedance is too high to trip the magnetic part of the trip?

The times allowed are a lot longer hence the thermal side ought to be fine.

Many of the yellow brick transformers don't have an internal impedance of less than 0.69 ohms so could never trip the magnetic part of the trip.

I'm not entirely sure that portable yellow bricks are covered by the IEE regs, RLV has a 5sec disconnection so the thermal side ought to suffice again. I can definitely trip a 10A MCB (as supplied) with a 3.3KVA (tool rated) brick transformer using a grinder as mine is a pain in the backside for it!

So either all the yellow brick transformers should be banned or having a high loop impedance line - line does not matter as long as there is an RCD. I have still not seen any yellow brick transformers with RCD protection it would be interesting to see if amendment 1 has given them an exemption?

Afaik, RLVs don't need RCD protection

 

[ericmark]

Reading the replies I now consider the magnetic part of the trip is to ensure the let-through energy can't raise to a value greater than the system can handle and where the line - neutral loop impedance is above the value which would allow under fault conditions the magnetic part of the trip to operate as long as there is RCD protection to cover the line - earth loop impedance there is no problem.

So calibrated loop impedance meter tests for MCB only protected circuits where exceeding the earth loop impedance could increase the tripping time from 0.01 seconds to 10 seconds is very important. However with fuses and RCD protected circuits having a calibrated loop impedance meter is not so important and one of the plug in units with a set of lights giving some indication as to the range of earth loop impedance would be good enough.

So for the DIY man who has fuses or RCD protection one of these

would do the job OK but where there is only MCB protection a calibrated one of these

is required. Or better.

As to if the tester shown about the cheapest I could find would work where there is RCD protection or even if the simple plug in tester would work where there is RCD protection I don't know. It may take out the RCD before it completes the test. Even my expensive loop impedance tester which should test without tripping the RCD does trip them on the odd time.

And of course they would also need a RCD tester. However looking at what a DIY guy is likely to buy I would say with the plug in cheap tester and pressing the test button on the RCD would reduce the likelihood of a dangerous fault due to extending a circuit to a reasonable level.

But there is a major problem where only a MCB protects the circuit as it would be near impossible for even a trained electrician to be sure the MCB would trip within a reasonable time under fault conditions without using expensive test equipment.

 

[bernardgreen]

The thermal trip will eventually trip on a prolonged overload and will ( should ) trip before the overloaded cable can become dangerously hot.

The magnetic trip operates rapidly on fault currents that are far in excess of overload currents and is probably intended to ensure rapid disconnection before weaker parts of the circuit collapse under a prolonged fault current.

Fault currents due to a shorted lighting cable could melt out a junction ( in a ceiling rose for example ) creating a second hot spot and reducing the fault current thus delaying the operation of a thermal trip. With only a slow acting fuse protecting a short circuited circuit other faults or damage could be created before the fuse ruptures.

 

[ericmark]

Fault currents due to a shorted lighting cable could melt out a junction ( in a ceiling rose for example ) creating a second hot spot and reducing the fault current thus delaying the operation of a thermal trip. With only a slow acting fuse protecting a short circuited circuit other faults or damage could be created before the fuse ruptures.

I agree with all you say. Down to a risk assessment. Most DIY people will carry on regardless. However we may from time to time make them think twice and they may get some basic test equipment. So to try and get a DIY guy to test is always hard. The older ones will have seen other people over the years doing many jobs without testing. Years ago all I had was a low ohm meter and insulation tester only in the last couple of decades have we used loop impedance testers and RCD testers.

So we have to try and persuade the DIY guy that the risks have changed and that's why today testing is so important.

The sudden jump from a 0.01 second to a 10 second disconnection time was what I used as the leaver to try and persuade them to test. However this was referred to as being a load on nonsense on another forum which made me rethink what I have stated in the past. I started to wonder if I was in fact wrong.

Seems from replies I was not far out with my thinking.

 

[bernardgreen]

Down to a risk assessment.

Which an experienced and competent person on site can carry out sucessfully. A person in an office writing a set of fit all regulations cannot perform a risk assessment of any real location. By creating a set of "fit all" regulations that must be obeyed the authorities may have removed the ability for risk assessments to be performed. For most situations the "fit" is probably acceptable but for many situations the "fit all" regulations will create risks that may not be recognised if a risk assessment is not ( cannot ) be performed on the design for the installation.

So we have to try and persuade the DIY guy that the risks have changed and that's why today testing is so important.

Have the risks really changed and if so how have they changed ?. With the advent of RCDs that operate on the difference between live and neutral the risks from "leaky" electrics have been reduced. MCB's with the magnetic trip reduce the risks from severe overloads.

I think the risks may have increased due to blind obeyance of the "fit all" regulations.

The sudden jump from a 0.01 second

is it really possible to get a 0.01 second disconnect when the half cycle time is 0.01 of a second ? Is the operating time measured from [a] the zero crossing point when there is no power to operate the trip the point on the rising sine voltage where there is enough current from V/z to trip the mechanism.

 

[ericmark]

Your words are very true. The requirements for electrical installations book was originally a guide. In fact it stated 114.1 the regulations are not law.

What has caused the problem is electricians who are members of a scheme have to follow the scheme providers rules which means the book are far as many are concerned is law.

I remember how all praises were sung over the voltage operated earth leakage trip only to see it outlawed latter.

There seems at the moment to be a problem with outside sockets on premises with caravans and the like parked very close. Doing a risk assessment there is no way one would earth a metal down spout to one system and a caravan within arms reach to another system yet the regulations if followed to the letter want you to do just that.

At one time it did state common sense should prevail but seems we have lost that now.

It is all too easy to blindly follow a rule book. But as electricians we hope we know what we are doing. However that's not true with DIY. How many times do you see some one who says they are competent asking a question which makes it plain they don't have the knowledge required to look after their own and others safety.

 

[bernardgreen]

I remember how all praises were sung over the voltage operated earth leakage trip only to see it outlawed latter.

In some ways the voltage operated is better. but it requires that there are no parallel paths to ground. The introduction of boilers and other appliances that created paths from the CPC to ground rendered them in-effective. They still held the CPC at less than 50 volts above ground but the fault current in the parallel path could be destructively high.

Doing a risk assessment there is no way one would earth a metal down spout to one system and a caravan within arms reach to another system yet the regulations if followed to the letter want you to do just that.

Yet they do flag up the problem in adjacent houses where one is TT and the other is TN-C-S ( PME ) and there are pipes linking the water systems ( or other links between the two CPCs ). They do flag that but do NOT flag up the external water tap that is copper all the way back to the MET which on a TN-C-S may be a good tingle voltage above ground. Which may be the reason why hand tools and mowers must be double insulated and NOT rely on ( or even have ) the CPC in the cable.

But as electricians we hope we know what we are doing. However that's not true with DIY.

True about the DIY and ( sadly ) also for some "house bashers" who have done the short course for electricians.