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timhoward

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To avoid distracting another thread...
@Julie helpfully said that an RCD can be used with an IT system to achieve disconnection after the second fault.

I'm not understanding how. If both faults are between transformer and a load it just becomes a parallel path with negligible resistance, and resistance to real earth would be higher, so to my thinking fault current to earth would be lower than 20ma.
If the faults are on opposing secondary legs then it becomes a dead short via the CPC and so much fault current would flow the over current device would operate.
Some silly mock-ups to illustrate my thinking:

If both faults between load and transformer: (low resistance used to avoid simulator complaining about loop)
IT system - how do RCD's ever work 1646745587244 - EletriciansForums.net
If both faults on either side of load:
IT system - how do RCD's ever work 1646745599649 - EletriciansForums.net

Am I correct so far, and if so what scenario would you get 20ma escaping to trip an RCD?
Many thanks.
 
confused.com.

a fault current is by definition a L-E or a L-N short circuit.

if we take the first case, L-E, then the fault current depends on 2 factors.

1, voltage, 2 resistance.

I+V/R. so assuming 230V and a resistance of upto 8K to earth, this will give an imbalance of the L and N in the RCD of up to 29mA, which will trip a 30mA RCD even without its weetabix.
 
Thanks @telectrix . That describes my understanding of most earthing systems.
I thought IT was different though. I think I need to back up to a more fundamental question.

On the secondary side of an isolating transformer you can touch either end of the winding and not get a shock as unlike TN setups neither end is connected to real earth and the current therefore isn't trying to flow via real earth and you.

My understanding (or error) is that joining one side of the secondary winding directly to a ground rod (for sake of argument) would not on its own cause any current to flow, it's instead just like joining a loose wire with nothing on the other end.
 
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There are two reasons for the rcd in an IT system, the first (which rather defeats the reason for an IT system) is on first fault.

In this case the system must be earthed via a high resistance, usually from a centre tap on the transformer (or via the earthy side), in this case any outgoing circuit once there is a first fault will see an unbalance. Hence trip on first fault.

The second, allows the system to continue, on a first fault.

In this case, each outgoing circuit has an rcd. In the event of the first fault on say circuit 1 as long as the capacitance of the system is small enough, although there may be a leakage current, it won't be sufficient to operated the rcd.

Now in the event of a second fault providing it's on a different circuit there will be a path between the two circuits and both rcd would see this unbalance and operate, most likely both operating in similar times disconnecting both faults.

There are issues with this.

Firstly, it is possible that only one rcd operates (quick enough to trip before the other does), thus you find one of the faults - it could be the first or second.

Secondly, this would not work if the faults were on the same rcd/circuit (but of course you have mandatory OCPD for that).

Thirdly in the event of high resistance faults, perhaps on the same phase (or neutral) there is no guarantee that sufficient current will flow.

Fourthly if both circuits are unloaded and the two faults occur on the same phase (or neutral), then no current will flow between the two circuits, it is only when one of the circuits becomes loaded that it would cause sufficient current to flow in the rcds.

Fifthly - further to the last point, there can be an unusual situation where the volt drop is the same on both circuits, for example (silly number alert) say there is a first fault 1 ohm down the neutral conductor which carries 5A - the voltage at this point would be 5V (with respect to the neutral bar). Now there is a second fault on a different circuit carrying 10A, but only 0.5 ohm down the neutral, again the voltage would be 5V (with respect to the neutral bar), since both these points are at the same potential, there wouldn't actually be any current between them, and no unbalance!!
(Think wheatstone bridge)


Because of these sorts of issues, and the fact that you need rcds on each circuit (a single up-front rcd like on tncs will not work), there is some debate about using simple rcds on IT systems, nevertheless they are permitted by the regs.
 
There are two reasons for the rcd in an IT system, the first (which rather defeats the reason for an IT system) is on first fault.

In this case the system must be earthed via a high resistance, usually from a centre tap on the transformer (or via the earthy side), in this case any outgoing circuit once there is a first fault will see an unbalance. Hence trip on first fault.
That depends on the reason for the IT system - though strictly speaking the moment you apply that earthing connection (even via a resistor) then it's no longer IT.
Make it a 55-0-55 secondary and you get the benefits of a low voltage, combined with the additional protection of the earth resistance backed up by the RCD. Think school labs where students either have no grasp of the dangers, or (more likely) are "inquisitive" - ah, the things we used to get away with when I were a lad 😂 Of course, teachers may be more clueless than the students in some cases 🙄
The second, allows the system to continue, on a first fault.

In this case, each outgoing circuit has an rcd. In the event of the first fault on say circuit 1 as long as the capacitance of the system is small enough, although there may be a leakage current, it won't be sufficient to operated the rcd.

Now in the event of a second fault providing it's on a different circuit there will be a path between the two circuits and both rcd would see this unbalance and operate, most likely both operating in similar times disconnecting both faults.

There are issues with this.

Firstly, it is possible that only one rcd operates (quick enough to trip before the other does), thus you find one of the faults - it could be the first or second.

Secondly, this would not work if the faults were on the same rcd/circuit (but of course you have mandatory OCPD for that).

Thirdly in the event of high resistance faults, perhaps on the same phase (or neutral) there is no guarantee that sufficient current will flow.

Fourthly if both circuits are unloaded and the two faults occur on the same phase (or neutral), then no current will flow between the two circuits, it is only when one of the circuits becomes loaded that it would cause sufficient current to flow in the rcds.

Fifthly - further to the last point, there can be an unusual situation where the volt drop is the same on both circuits, for example (silly number alert) say there is a first fault 1 ohm down the neutral conductor which carries 5A - the voltage at this point would be 5V (with respect to the neutral bar). Now there is a second fault on a different circuit carrying 10A, but only 0.5 ohm down the neutral, again the voltage would be 5V (with respect to the neutral bar), since both these points are at the same potential, there wouldn't actually be any current between them, and no unbalance!!
(Think wheatstone bridge)
But, none of those issues will create a safety risk. In all cases, either a device trips or there isn't a safety issue - assuming all the usual precautions like enclosures that keep fingers out and insulation that insulates.
Obviously, if someone opens up a cabinet and sticks their fingers across two line connections (arguably, you don't have L & N in a 2 wire It system, just L1 and L2) then they'll get a shock just as they would with any other supply type.
But as you point out, on a first fault, the system keeps going - which may be justification in itself.
As I mentioned in the other thread, IT systems are common in marine environments for the "keep going on first fault" feature - though I suppose another way of looking at it is avoiding the "bang and magic smoke" (along with disruption to the power system) before the OCP disconnects the circuit. But then they have (typically) automatic monitoring systems - the Bender kit can both tell you there's a fault, and tell you where it is if you install sufficient units in the right places.

But the critical thing is that there is a system or process in place to identify when that first fault occurs so it can be fixed. If it's ignored, then the system is no more resilient than if it had not been IT in the first place.

I've recently had reason to look into how the Bender EFM (Earth Fault Monitoring) systems work - it's both quite simple and quite elegant. Master unit impresses an earth fault current onto the system - think about if you replace the resistor in your first example (resistively earthed centre tap) with a signal generator. At each monitoring point (e.g. outgoing way in a switchboard) there's a detector unit connected to a CT around all the outgoing conductors (excluding CPC of course).
If there's no earth fault, all that happens is that the master unit pushes the voltage phasor diagram around relative to earth. If there's a fault, a corresponding current is detected by the CT, correlated with the impressed signal, and an alarm raised - e.g. by a signal into the SCADA.
However, it gets "more interesting" when you add a load of high power variable speed drives - adding capacitance to the system, and capable of creating high frequency rotating faults. Think about what happens if you inject (say) 440V at a few hundred (or more) Hz into filter caps designed to be across 240V at 50 Hz because one side of the DC link in a drive (with active front end) has an earth fault.
 
That depends on the reason for the IT system - though strictly speaking the moment you apply that earthing connection (even via a resistor) then it's no longer IT.
That is another interesting question: When is an IT supply no longer IT?

Some systems have an earthing impedance to deliberately limit the PFC to something manageable for disconnection, but otherwise you would say they were TN.

Other than things like shaver sockets or some specialised lab supplies I suspect most IT systems have some earthing impedance to avoid strange capacitive effects taking over and to facilitate leakage monitoring. I always assumed your IT choice was to avoid failing on first-fault so such impedance would be high enough that protection did not trip (or impedance roast).

So it kind of negates the point of an RCD as Julie said!
I've recently had reason to look into how the Bender EFM (Earth Fault Monitoring) systems work - it's both quite simple and quite elegant. Master unit impresses an earth fault current onto the system - think about if you replace the resistor in your first example (resistively earthed centre tap) with a signal generator. At each monitoring point (e.g. outgoing way in a switchboard) there's a detector unit connected to a CT around all the outgoing conductors (excluding CPC of course).
Ah, cunning. I had wondered about them and has assumed a DC test but that would make it hard to follow where any leak was going.
If there's no earth fault, all that happens is that the master unit pushes the voltage phasor diagram around relative to earth. If there's a fault, a corresponding current is detected by the CT, correlated with the impressed signal, and an alarm raised - e.g. by a signal into the SCADA.
However, it gets "more interesting" when you add a load of high power variable speed drives - adding capacitance to the system, and capable of creating high frequency rotating faults. Think about what happens if you inject (say) 440V at a few hundred (or more) Hz into filter caps designed to be across 240V at 50 Hz because one side of the DC link in a drive (with active front end) has an earth fault.
Yes, for most modern systems the capacitance is going to be a problem, more so if you use a higher frequency to probe the system. But then CT get bulky if you want to get good LF responses, or you use something far more expensive like Hall-effect or flux gate magnetometers (as type B RCD do).
 
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Yes, for most modern systems the capacitance is going to be a problem, more so if you use a higher frequency to probe the system.
I wonder if they could probe with two frequencies to see if any leakage is proportional to frequency (i.e. capacitance) or fairly constant (i.e. resistive fault)?
 
That is another interesting question: When is an IT supply no longer IT?

Some systems have an earthing impedance to deliberately limit the PFC to something manageable...

If the system is DIRECTLY earthed, then it is type T... (TN.. OR TT). Basically direct, or via a low impedance means fault levels being ideally greater than full load current.

If it is NOT directly earthed - that being unconnected or connected via a high impedance, then it is type I... (IT).

Essentially if the fault current is very small due to the earthing arrangement it is IT.
 
That depends on the reason for the IT system - though strictly speaking the moment you apply that earthing connection (even via a resistor) then it's no longer IT.
.....

No TN.. and TT are defined as being earthed directly, whilst IT is not directly connected, the principle being the impedance to earth being substantially high and very reduced fault currents - much less than typical full loads.

All systems are basically connected to earth the difference being is it direct, or indirectly via high impedance -intentionally, or unintentionally via parasitic capacitance.

...
Make it a 55-0-55 secondary and you get the benefits of a low voltage, combined with the additional protection of the earth resistance backed up by the RCD.

...
That is or isn't a totally different question than asked by the OP.
The question asked is how RCD works in an IT system, if your 55-0-55 transformer is earthed directly then it is again an TT or TN system, if not directly earthed then indeed it would be IT, it is the method of earthing that defines the earthing type, not the voltage or winding arrangement.


....either a device trips or there isn't a safety issue - assuming all the usual precautions like enclosures that keep fingers out and insulation that insulates.
....
arguably, you don't have L & N in a 2 wire It system, just L1 and L2)
....
But as you point out, on a first fault, the system keeps going - which may be justification in itself.
....

No, a dangerous situation can arise due to the single fault, as this basically becomes similar to TT , one outgoing circuit can have a fault - effectively connecting L1 to earth, thus every other circuit (on other phases) has a potential of 400V to earth rather than 230V, insulation failure in contact with a seperate earth (effectively different from the first earth) means that the "earthed" body on circuit 1 has a difference in potential to the "earthed" body on the other circuit. - essentially there is 400V between two "local earths" - way more than usual. This generally doesn't apply on ships as the metalwork is fairly continuous throughout, superyachts on the other hand end up in a more dangerous situation due to the higher voltages to earth (for example on "One more toy" - which although is TNC has other areas without the continuous earth/neutral due to GRP construction presenting similar situation to an IT system with a single fault.)

I agree about no real N on an unearthed two wire system, I just retained the terminology as it's more familiar to most.

....
But the critical thing is that there is a system or process in place to identify when that first fault occurs so it can be fixed. If it's ignored, then the system is no more resilient than if it had not been IT in the first

Absolutely, and actually can represent a higher risk do to the higher voltages to earth.
 
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No TN.. and TT are defined as being earthed directly, whilst IT is not directly connected, the principle being the impedance to earth being substantially high and very reduced fault currents - much less than typical full loads.
OK then, where does the line lie between "IT with a highish impedance to earth" become "TN or TT with a highish impedance to earth" ? There are the extremes - something resembling a "very low" impedance is TN or TT, something resembling a "very high" impedance is IT. When "very low" changes to just "low", or "very high" changes to just "high" - where is the threshold ? Somewhere in between those extremes you will change from one to the other.
No, a dangerous situation can arise due to the single fault, as this basically becomes similar to TT , one outgoing circuit can have a fault - effectively connecting L1 to earth, thus every other circuit (on other phases) has a potential of 400V to earth rather than 230V
We were talking about a two wire 240V IT system. Earthing L1 or L2 will only create 240V between the other line and earth.
insulation failure in contact with a seperate earth (effectively different from the first earth) means that the "earthed" body on circuit 1 has a difference in potential to the "earthed" body on the other circuit.
That's not unique to IT systems - and why we have bonding to avoid having different "earth" potentials within the bonded zone.
 
OK then, where does the line lie between "IT with a highish impedance to earth" become "TN or TT with a highish impedance to earth" ? There are the extremes - something resembling a "very low" impedance is TN or TT, something resembling a "very high" impedance is IT. When "very low" changes to just "low", or "very high" changes to just "high" - where is the threshold ? Somewhere in between those extremes you will change from one to the other.
Isn't the distinction high impedance by deliberate design (IT) versus high impedance through natural environmental circumstances (TT)?
 
But still, at some point you have to decide what protective/control measures are needed. Then it doesn't rally matter whether it's deliberate or imposed ...
 
But still, at some point you have to decide what protective/control measures are needed. Then it doesn't rally matter whether it's deliberate or imposed ...
Well it does, if you have an earth loop impedance of 5k ohm, that tends to be a fault condition on a TT and TN system, which needs to be fixed so the usual protection (std RCD and OCPD) will work, whilst that would likely be somewhat low - a potential fault meaning a first fault on an IT system.

The distinction is clear in the regs, TN & TT systems are directly earthed implying similar fault levels to short circuit faults or at the very least similar fault currents to flc.

IT on the other hand are not directly connected, and therefore have fault currents significantly lower than flc.

If the impedance is between these, it represents an unexpected situation outside of the design of either system.
 
The distinction is clear in the regs, TN & TT systems are directly earthed implying similar fault levels to short circuit faults or at the very least similar fault currents to flc.

IT on the other hand are not directly connected, and therefore have fault currents significantly lower than flc.
That is not really the case though as even a domestic TT system can have currents to tens of amps on each final circuit, but a 200 ohm rod so PFC below 1A. I guess the defining point would be not the FLC but the operating point of the designed protection system (so in such a TT case the RCD would be tripping typically for faults below 7.6k ohm at most).

Which kind of returns to the original discussion about IT systems and that they are normally designed to continue under single fault conditions, and ideally safely (as far as practical) so you might be looking at earthing impedances of tens of k ohm if trying to keep the 2nd fault touch shock current below, say, 10mA (assuming that is even possible depending on filter caps, etc).
 
That is not really the case though as even a domestic TT system can have currents to tens of amps on each final circuit, but a 200 ohm rod so PFC below 1A. I guess the defining point would be not the FLC but the operating point of the designed protection system (so in such a TT case the RCD would be tripping typically for faults below 7.6k ohm at most).

Which kind of returns to the original discussion about IT systems and that they are normally designed to continue under single fault conditions, and ideally safely (as far as practical) so you might be looking at earthing impedances of tens of k ohm if trying to keep the 2nd fault touch shock current below, say, 10mA (assuming that is even possible depending on filter caps, etc).

Yeah, I am not sure how to word it though, TT systems still have a decent amount of current in the order of amps (minimum should be above 1A ) whilst IT systems are usually 2 or more orders of magnitude smaller than this - usually 10mA or less.

What I am trying to say, is that an earth fault on a TT OR TN system is a reasonable level to detect - you could use a 100mA rcd and still have fault current in excess of 10x the trip setting, these are easy and almost foolproof systems to be able to detect an earth fault.

This is a vastly different situation to an IT system where the current, on first fault will be in the order of mA or less.

There isn't really a cross over where you might have an IT system having a first fault current of 100mA, or a TT system of also 100mA (200 ohm is the expected minimum).

Detection of first faults is likely to have to be much less than 1mA unless you are forcing a first trip situation by upping this to 10mA or so.
 
Where are these dangerous touch voltages coming drom
... if trying to keep the 2nd fault touch shock current below, say, 10mA
That implies a situation where at least some items are neither Class II nor earthed/bonded - which is not generally permitted for obvious reasons.
Yes, there will some cases where the IT system is there to directly control touch currents - shaver sockets come to mind. But in the context of the OP's IT supply to a distribution panel, all the normal protective measures (principally bonding via CPCs) are (or should be) there to control touch voltages/currents.
 
My thoust was as much a "if you need to chose a single-fault threshold" sort of point than seeing big IT supplies as touch-safe.
 

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