That's a good point. Medical iso xfmrs are usually less than 120pF stray capacitance.
Driving a new ground spike
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Bo Deadly
Well-known member
I didn't realize before that the ground electrode system bus is not connected to neutral at all in this diagram. So it's not just fault currents from the RCBOs that will go to the ground rod, it's fault currents of all breakers.
Rob Flinn
Well-known member
Here's a pic of the beast. It's so heavy it puts me absolutely on the limit to pick it up. Question, it there a way to measure the stray capacitance without being in a physics lab ?
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Not very easy, because the actual capacitive effects are capacitance from primary to core, capacitance of secondary to core and capacitance between primary and secondary. The specified capacitance is the latter, but the others are larger.
What are fault currents from breakers?
My understanding is that fault currents are related to circuits.
RCD and GFCI can operate properly with only two active connections (line and neutral). It doesn't matter where stray currents flow, only that they are not flowing in the correct conductors.
JR
JR
That's why I implied that they don't flow from the breakers.
Bo Deadly
Well-known member
Fault current that is ultimately sourced from a breaker. So it's not just currents sourced from the RCBOs that will fault to the ground electrode system. Current sourced the MCBs also will fault to the ground electrode system. Meaning there's nothing special about RCBOs in this respect. But I'm getting the feeling I'm the only one that missed that so ... carry on.
Matt Syson
Well-known member
Had a strange day here. Trying to #=unravel a mystery for a friend. Electrics fine for 2 years but a lightning strike in the locale a few days ago. NOW ONE of the 30ma tripping RCDs is acting strangely.
the premises has 4 30 milliamp breakers and a 'master 500 Milliamp (electric company master breaker/switch.
The 500 milliam main trips after a couple of minutes if ONE of the 30 MilliampRCDs is switched ON (but the load breakers are OFF so ONLY the NEUTRAL is actually connecting. With 230 Volts incoming to the RCD it will not 'trip when the test button is pressed. Bought a new one and that also won't trip when tested. Brought the original RCD home and it works fine on the bench with 230 Volts applied.
Cooker is fed PRE the 30 milliamp RCDs (and (incorrectly so is the socket used for kettle).
Now trying to work out how wiring that has been working for 2 years is now 'incorrect as it seems the Neutrals (wired to UK 'code') are crossed somewhere as it seems something is picking up a Neutral from the 'wrong' RCD. WHY the 'fault' takes about 1 or 2 minutes to trip the master 500 milliamp RCD is just part of the mystery. Must be 'wine time'.
Identifying the 40 or more blue Neutrals and pairing them with respective lives is a project for another day. The Neutral is wandering around about 3 Volts above 'Earth (well the bar with some green/yellows on it) but the limit is 10 Volts to allow for resistive drops in the Neutral from where the electric company bond them.
the premises has 4 30 milliamp breakers and a 'master 500 Milliamp (electric company master breaker/switch.
The 500 milliam main trips after a couple of minutes if ONE of the 30 MilliampRCDs is switched ON (but the load breakers are OFF so ONLY the NEUTRAL is actually connecting. With 230 Volts incoming to the RCD it will not 'trip when the test button is pressed. Bought a new one and that also won't trip when tested. Brought the original RCD home and it works fine on the bench with 230 Volts applied.
Cooker is fed PRE the 30 milliamp RCDs (and (incorrectly so is the socket used for kettle).
Now trying to work out how wiring that has been working for 2 years is now 'incorrect as it seems the Neutrals (wired to UK 'code') are crossed somewhere as it seems something is picking up a Neutral from the 'wrong' RCD. WHY the 'fault' takes about 1 or 2 minutes to trip the master 500 milliamp RCD is just part of the mystery. Must be 'wine time'.
Identifying the 40 or more blue Neutrals and pairing them with respective lives is a project for another day. The Neutral is wandering around about 3 Volts above 'Earth (well the bar with some green/yellows on it) but the limit is 10 Volts to allow for resistive drops in the Neutral from where the electric company bond them.
moamps
Well-known member
I would suggest that you carefully check whether there are black carbonized traces of the passage of the arc from the line wires to ground or neutral that can remain conductive for a long time. Also some RCDs have an active circuitry that may be internally damaged (MOVs inside etc.) by lightning. If the test switch does not activate the RCD, it is a clear sign that either the RCD is defective or one wire/connection is missing, live or neutral.
presumably line and neutral are connecting to the RCD, if I understand no load is connected downstream but utility breaker (RCD?) still trips.
no bueno... The trip test is shunting current around the sense loop to create imbalance. Not test tripping is a huge red flag warning
If the 500mA mains trips without loads connected that suggests to me the problem may be close to that and/or upstream.
Lightning is tens of thousands of voltage so will find it's own path.. it can punch holes in insulation but they are generally inconsequential later at modest mains voltage. I've seen conductive carbon tracks in automobile distributor caps (also tens of kVolts) not so much in house wiring.
I would stop looking deeper if it can't even support a good RCD with no load.
JR
No. in most cases, the fault current comes fom the live wire touching a earthed body. Unless seriously damaged, the breaker does not leak current.
Very often, the fault current doesn't flow to the ground rod (spike). When you touch the live wire and you're standing on a tile floor, current goes to earth in a very diffuse way that completely ignores the ground rod.
Of course.
To me it's a sign the leak is pretty low, so the magnetic does not detect, but the thermal does.
Matt Syson
Well-known member
To me it's a sign the leak is pretty low, so the magnetic does not detect, but the thermal does.
Thanks a good thought BUT as the 500mA breaker is happy to supply the cooker AND kettle (about 8 Amps each Both separately and together, for the duration of the test, I can't see it being the thermal part as such. The phone/modem/internet gear was not damaged (a frequent issue in these parts) which involves taking your smoking 'Livebox' (modem supplied by Orange Telecom) to the local Orange dealer who then painstakingly checks the serial numbers and gives you a new one.
I need to return withdifferent 'test' gear such as a 15 Watt mains bulb so that the presence of voltage from a suitanly low impedance (normal working) can be checked as DVM s don't really reflect the SOURCE impedance. The same sort of reason that LED lamps can 'flash' when the capacitance of either live or neutral can pick up sufficient voltage by capacitance over a long cable t#run so that the mini switcher supply inside the LED lamp suddenly wakes up every few minutes sufficient to flash the LEDS before it discharges.
Regardiong that isolation transformer. I suppose it cOULD act as a form of serious surge voltage limiter in that diven a surge of a couple of KV (the core would (temporarily) saturate so limiting the secondary side surge to ??? hundreds of Volts. It will of course increase the IMPEDANCE of the supply reaching your studio so the HiFi brigade will criticise you for not having a direct, infinitely low impedance supply direct from the power station. This will of course muddy your lower mids and increase HF flutter.
Thanks a good thought BUT as the 500mA breaker is happy to supply the cooker AND kettle (about 8 Amps each Both separately and together, for the duration of the test, I can't see it being the thermal part as such. The phone/modem/internet gear was not damaged (a frequent issue in these parts) which involves taking your smoking 'Livebox' (modem supplied by Orange Telecom) to the local Orange dealer who then painstakingly checks the serial numbers and gives you a new one.
I need to return withdifferent 'test' gear such as a 15 Watt mains bulb so that the presence of voltage from a suitanly low impedance (normal working) can be checked as DVM s don't really reflect the SOURCE impedance. The same sort of reason that LED lamps can 'flash' when the capacitance of either live or neutral can pick up sufficient voltage by capacitance over a long cable t#run so that the mini switcher supply inside the LED lamp suddenly wakes up every few minutes sufficient to flash the LEDS before it discharges.
Regardiong that isolation transformer. I suppose it cOULD act as a form of serious surge voltage limiter in that diven a surge of a couple of KV (the core would (temporarily) saturate so limiting the secondary side surge to ??? hundreds of Volts. It will of course increase the IMPEDANCE of the supply reaching your studio so the HiFi brigade will criticise you for not having a direct, infinitely low impedance supply direct from the power station. This will of course muddy your lower mids and increase HF flutter.
Of course. What was I thinking?
RCD/GFCI are magnetic but their trip time is a function of current imbalance...
I don't have anything useful to add.
JR
I don't have anything useful to add.
JR
Matt Syson
Well-known member
The 'fact' that a standard 80 Amp 30mA trip RCD when fed with 230 Volts on it's input fails to trip when pressing the test button with only the Neutral pole of the switched side connected to anything then another brand new (bought at lunchtime) ALSO fails to trip when 'Tested' is so bizarre. especially as when I got the original home and wired it to a mains plug, it 'tripped' like a good un'. even when swapping the 'input' and output' connections it STILL worked. I hate the way that the various manufactuereres have inputs at the top and others are at the bottom AND the wire holes are not aligned so different makes require a 'dog leg' in the commoning bar.
Well I have established that the sockets in his kitchen are not fed from a 30 Amp RCD so that WILL need sorting out.
Well I have established that the sockets in his kitchen are not fed from a 30 Amp RCD so that WILL need sorting out.
Matt Syson
Well-known member
Thanks JR. The whole thing is so strange because all at least appeared to be fine for 2 years and without changing any wiring at all but during a thunderstorm one 30mA RCD fails to act correctly and the 500mA 'master' breaker trips only when 4 circuits worth of Neutrals are added to the 'mix.
Actually I might have it. IF one of the 4 Neutrals is actually going to ground via a couple of K Ohms or maybe less, it COULD be causing ??? Milliamps to flow in addition to the normal balanced current taken by the rest of the system. The Neutrals are all sitting at about 3 Volts above 'ground' (measured to earth terminals in the breaker box. So about 6 Ohms so more meauring is necessary THEN rip the house apart to find where each cable actually goes.
Actually I might have it. IF one of the 4 Neutrals is actually going to ground via a couple of K Ohms or maybe less, it COULD be causing ??? Milliamps to flow in addition to the normal balanced current taken by the rest of the system. The Neutrals are all sitting at about 3 Volts above 'ground' (measured to earth terminals in the breaker box. So about 6 Ohms so more meauring is necessary THEN rip the house apart to find where each cable actually goes.
I would be tempted to unplug all the appliances too... I have seen faulty protection MOVs in cheap outlet strips dumping current from line to safety ground.
I predict it will be the last thing you check.
JR
I predict it will be the last thing you check.
JR
neil.johnson
Well-known member
My point, perhaps not blunt enough, was that posting information about UK electrical safety from a site outside of the UK (in this case Serbia, but would be equally at issue were it USA, France, Germany, wherever) from an unknown organisation with content of dubious origin and correctness, doesn't really help a thread on a safety-related topic. Each country has its own electrical codes for a variety of reasons, so you cannot compare safety earthing systems unless treading very carefully.
Yes, I know this is the internet and every man and his dog can express whatever they feel like saying, no matter whether it is correct or not. Nowhere in that posted article could I find the name, affiliation, or qualifications of the author.
As for the specific diagram in post #132, that shows a "TT" system as it is called in the UK. But there are two other earthing systems (TN-S and TN-C-S), and also note that the diagram posted does not show the complete earth circuit as it fails to show the supply source transformer with its Neutral connected to ground as well.
As others have either explicitly stated or alluded to, current doesn't just go down the earth rod by itself. Current flows in a circuit, and it will find the lowest impedance path back to where it came from (impedance, because mains supplies are mostly AC
). The only function safety earth has to do is provide a lower impedance path for any mains fault currents to flow along instead of you. And one side-effect of that is that, here in the UK, the protective earth is allowed to rise to about 40V under fault conditions. That's not enough to electrocute (kill) you, and the intention is that that situation won't last too long as an RCD or other protection device will trip in less than a second or so (hopefully much faster!).Neil
