Overload, Overcorrect and ADS...

[junksamiad]

Hi all,

probably a couple of silly questions here, I know. I just need some clarification if possible.

Firstly. Is there a difference between overload and overcurrent? I've always assumed they are the same thing but recently I have been considering this more and I'm reasoning that overload could relate, for example, to where current is flowing that is more than the cable is rated to carry? In contrast, overcurrent could relate to large current caused by a short circuit?

Secondly, when we talk about ADS, would ADS cover overload/overcurrent, and fault current, or does ADS only apply to fault current (short circuit or earth fault).

If any of the question appears confusing then hopefully you can easily pickup on how I may be thinking about this in the incorrect way and help me out.

Thanks again for any help.

 

[telectrix]

first, overload and overcurrent are basically the same. it's the magnitude thar differentiates between an overload and a fault current.
second, ADS is nothing to do with overload, it's more concerned with fault current. it relares to a short circuit between live parts and exposed coductive parts ( e.g. a live conductor in a metal back box shorting to the box.).

 

[junksamiad]

first, overload and overcurrent are basically the same. it's the magnitude thar differentiates between an overload and a fault current.
second, ADS is nothing to do with overload, it's more concerned with fault current. it relares to a short circuit between live parts and exposed coductive parts ( e.g. a live conductor in a metal back box shorting to the box.).

Thank telectrix. I thought as much with the ADS thing, as I'm right in thinking overload potentially will not trip a protective device instantly anyway would it? I'm understanding that a higher current (than the device is rated at) could cause the protective device to sit there for some time or at least until the fault current that the device will trip at is reached (according to the time/current graphs in BS7671)? Is that correct thinking?

 

[telectrix]

Thank telectrix. I thought as much with the ADS thing, as I'm right in thinking overload potentially will not trip a protective device instantly anyway would it? I'm understanding that a higher current (than the device is rated at) could cause the protective device to sit there for some time or at least until the fault current that the device will trip at is reached (according to the time/current graphs in BS7671)? Is that correct thinking?

sort of right. asmall overload will not trip the device for a certain time, in accord with the time/current graphs. the higher the current, the faster it will trip.

 

[Julie.]

As described above - the collective term 'overcurrent' applies to both overload and fault current.

ADS is provided via operating due to fault current. - ADS itself is part of protecting against electric shock, so a fault can cause voltages to appear in places rendering a shock risk, however an overload (22A in a 20A cable for example) does not really present any additional risk of shock. - in can however contribute to overheating/fire/deterioration in the lifespan of equipment.

So overload protection isn't actually part of ADS.(as part of protection against electric shock)

Unless using any other of the 4 protective measures, if you use ADS then you MUST have fault current protection, and it must be at the upstream end - so a fuse protecting a cable must be at the source end, not the load end.

If there is no possibility of an overload (such as a feeder providing a fixed load) then overload protection can be omitted.

If there is overload protection provided for a load on a single feeder, then it could be placed at the load end (it could make more sense there) or at the source end.

An example may be a 32A MCB/RCBO on a 20A cable (as long as the Zs is low enough to provide ADS for fault current) :

If this is providing say a fixed 18A load (shower, hob etc) - no overload is needed.

If this is providing say a socket outlet where people could add loads in excess of 20A - some form of overload protection is required (and can be fitted at either end of the cable although it makes sense to fit it at the source end by changing the MCB).

If this is providing a fixed motor (which can obviously be physically overloaded) - some form of overload protection is required (and can be fitted at either end of the cable although in this case it makes sense to fit it at the load end of the cable in the motor starter as this is the usual place).

 

[junksamiad]

As described above - the collective term 'overcurrent' applies to both overload and fault current.

ADS is provided via operating due to fault current. - ADS itself is part of protecting against electric shock, so a fault can cause voltages to appear in places rendering a shock risk, however an overload (22A in a 20A cable for example) does not really present any additional risk of shock. - in can however contribute to overheating/fire/deterioration in the lifespan of equipment.

So overload protection isn't actually part of ADS.(as part of protection against electric shock)

Unless using any other of the 4 protective measures, if you use ADS then you MUST have fault current protection, and it must be at the upstream end - so a fuse protecting a cable must be at the source end, not the load end.

If there is no possibility of an overload (such as a feeder providing a fixed load) then overload protection can be omitted.

If there is overload protection provided for a load on a single feeder, then it could be placed at the load end (it could make more sense there) or at the source end.

An example may be a 32A MCB/RCBO on a 20A cable (as long as the Zs is low enough to provide ADS for fault current) :

If this is providing say a fixed 18A load (shower, hob etc) - no overload is needed.

If this is providing say a socket outlet where people could add loads in excess of 20A - some form of overload protection is required (and can be fitted at either end of the cable although it makes sense to fit it at the source end by changing the MCB).

If this is providing a fixed motor (which can obviously be physically overloaded) - some form of overload protection is required (and can be fitted at either end of the cable although in this case it makes sense to fit it at the load end of the cable in the motor starter as this is the usual place).

Thanks Julie, a very comprehensive answer. So I see now, ADS is there to protect from electric shock. So it will apply to earth faults, hence the RCD. Will this apply to short circuit faults as well then? Or are they not determined a shock risk as essentially they are a circuit overload of current and not sending current to earth?

 

[Julie.]

Thanks Julie, a very comprehensive answer. So I see now, ADS is there to protect from electric shock. So it will apply to earth faults, hence the RCD. Will this apply to short circuit faults as well then? Or are they not determined a shock risk as essentially they are a circuit overload of current and not sending current to earth?

Short circuit is part of ads protection, the likely cause is a nail/screw through the cable or similar presenting a shock risk.

Overload is a small magnitude thing, whilst fault current is expected to be higher - fault means a fault in the system, overload is using a bit more than you should!

Check out sections 41 & 43 in the standard

 

[pc1966]

ADS is for fault cases: shock, fire, and equipment damage risks, and are:

• An OCPD that detects high fault currents (and cares not where they go to)
• An RCD that will detect small currents going astray (most probably to Earth, but could be to another circuit's neutral, etc)

The original means of protection is the OCPD (fuse/MCB) and that is almost always needed anyway, and more recently (in electrical history terms) RCD became cheap enough to be more or less standard fit on most final circuits now for additional protection.

An OCPD will not, on its own, protect you from shock as the trip level is way above biological damage levels. But when combined with bonding/earthing of class I equipment, etc, is provides a means of limiting the time where a high voltage could be presented on any touchable conductive object. For example, if your R1=R2 then under a hard fault you would see Uo/2 on the "Earthed" metalwork, above the 50v that is generally acceptable, so the exposure time has to be limited to a fraction of a second for safety.

Similarly not all RCD will protect directly against shock, for example, a 300mA S-type incomer will easily allow deadly time/current exposure, but they might be doing so by ADS when you have too high an earth impedance for the OCPD to trip. The classic example is a TT supply when the local earthing rod impedance is typically in the tens of Ohms and would be unlikely to trip even a 6A MCB sufficiently fast to reduce fault exposure times to acceptable levels.