what are the insulation classes

[hsn93]

hi,

https://en.wikipedia.org/wiki/Appliance_classes


Class I:

These appliances must have their chassis connected to electrical earth (US: ground) by a separate earth conductor (coloured green/yellow in most countries, green in India, USA, Canada and Japan). The earth connection is achieved with a 3-conductor mains cable, typically ending with 3-prong AC connector which plugs into a corresponding AC outlet. The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that if a fault occurs the supply will be removed automatically (this is sometimes referred to as ADS = Automatic Disconnection of Supply).

A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. If large enough, this current will trip an over-current device (fuse or circuit breaker (CB)) and disconnect the supply. The disconnection time has to be fast enough not to allow fibrillation to start if a person is in contact with the casing at the time. This time and the current rating in turn sets a maximum earth resistance permissible. To provide supplementary protection against high-impedance faults it is common to recommend a residual-current device (RCD) also known as a residual current circuit breaker (RCCB), ground fault circuit interrupter (GFCI), or residual current operated circuit-breaker with integral over-current protection (RCBO), which will cut off the supply of electricity to the appliance if the currents in the two poles of the supply are not equal and opposite.

this is actually what i know!

earth the chassis of the product.
and a failure or earth leakage will trigger the RCD in the electric panel.

Class 0I
Electrical installations where the chassis is connected to earth with a separate terminal, instead of via the mains cable. In effect this provides the same automatic disconnection as Class I, for equipment that otherwise would be Class 0.

ok what is the difference electrically between this and Class I ?! both are connected to earth !
but i guess the difference is that this would still be earthed even if the cable plugged out ?

Class II
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).

The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having at least two layers of insulating material between live parts and the user, or by using reinforced insulation.

In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol: ⧈ (a square inside another square).

Insulated AC/DC power supplies (such as cell-phone chargers) are typically designated as Class II, meaning that the DC output wires are isolated from the AC input. The designation "Class II" should not be confused with the designation "Class 2", as the latter is unrelated to insulation (it originates from standard UL 1310, setting limits on maximum output voltage/current/power).

i think most (mobile chargers) are just plastic right? so no earth chassis stuff..
plastic enclosures is one insulating material what is the second  insulating materials of the cell-phone chargers? is it the air between the wires/PCB and the plastic?

so what distinguishes (Class I) and (Class II) is that the chassis is insulation material (so not required to PE chassis for Class II) ?

dc output wires are isolated from the ac input

are we talking about galvanic isolation provided by the transformer or just casing and chassis isolation from earth?

i cant understand what will be the alternative of this anyway
non isolated switching and a filter?

A Class III appliance is designed to be supplied from a separated/safety extra-low voltage (SELV) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock. The extra safety features built into Class I and Class II appliances are therefore not required. For medical devices, compliance with Class III is not considered sufficient protection, and further more-stringent regulations apply to such equipment.

i really dont see the picture, whats the difference between Class III and Class II.

The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock. The extra safety features built into Class I and Class II appliances are therefore not required

isnt that the case of Class II anyway  ??







and finally what are the differences between these (SELV , PELV, FELV) i tried to research about it..
but i cant find the answer that really answers my question (most of them are no technical details)


even the standard EN6114:



    what is the difference ?




doesn't make sense if definition of SELV = PELV !!?


if the (PELV circuit is earthed and protective screening, not necessary basic insulation) .. inst that Class II again





can i know few examples of what makes a device being in which category :
(class I -> Class II)
(class II -> Class III)
(SELV -> PELV)
(SELV -> FELV)
(PELV -> FELV)

[T3sl4co1l]

Class 0I uses a separate ground path.  Example: semi-permanently installed equipment bonded with a ground spike.

Class II I'm not sure what counts as layers, possibly it's air inside the enclosure plus the enclosure thickness.  A failure of the enclosure isn't obvious, would that be cracking or fragmenting or completely falling off?  I'm not sure offhand how that's counted (and I'm sure it varies by standard).

Class III is SELV to SELV, so there's no need for insulation.  An example product might be a DC-DC converter module, with functional insulation rated something low, like 100V.  Such a device might be used where isolation is needed but high voltage is not, like breaking ground loops in communication systems, like Modbus (e.g., RS-485 crossing throughout a facility, so that it picks up far more than the +/-7V or whatever common mode range the receivers can handle).

Might also be used for PELV or TNV circuits, depending on rating.  Ethernet for example, is transformer coupled, with, functional or basic type insulation, I'm not sure which; and a 1.5kV isolation rating.  This covers ESD (which might be a 150pF 8kV source, which gets divided into a 1nF capacitor, reducing it to about 1kV peak); but does not very well cover induced lightning or mains transients (2.5kV+), so a long run (say, between facilities) or an accidentally cross-wired circuit (e.g. Ethernet cable gets nicked and touches mains) might not do so well.

Using such a device, on a circuit expected to have transients, may require protective devices to prevent their breakdown, and that then requires consideration of where the protection current flows.

For example, you can put MOVs or GDTs on a telephone line (which makes long runs between facilities -- expected to experience induced lightning surge), but the telephone set either needs to be grounded (so the fault current can be dissipated safely) or double insulated (so there's no risk to the user).

Regarding S/PELV and such: the CAPITALIZED TERMS are all described in the front section of these standards.  Just look up the rest of the standard for definitions.  Very boring reading, but eh.

SINGLE FAULT is one such term.  A ground fault for example, is where a ground conductor has failed (broken).  This might happen through injury or corrosion of a cable or connector.  Obviously, if the device depends on sinking faults to ground, they will no longer be safe when this happens.  This would violate the single-fault-tolerant property of SELV.  The above hypothetical telephone, with a metal chassis and safety ground connection, would not be safe if it had just one ground wire.

Which, we have actual examples of these: the old fashioned telephone booth, mostly made of metal.  I assume they are (semi-)permanently installed in such a way that grounding is redundant, therefore a single failure in any given connection does not lead to unsafe operation.  (They may also be subject to standards much more stringent than e.g. IEC 60950-1 or the like; I have no idea.)

So, if you're just reading bits and pieces of standards, yeah it's not going to make much sense, go look up the whole standard if you can find it (or if you NEED it, don't be an idiot, BUY IT!).

Most of IEC has been adopted -- and made public -- by India.  I don't know about EN standards.  There may be compatible or similar standards in other systems that can be found.

Tim

[Neilm]

Why do you think SELV = PELV? The difference is explained in the first part you posted. From what you posted (I am not too familer with PELV) SELV includes protection to other circuits so the designer has to account for earth faults on anything that can be plugged in to it. PELV does not require this.

I am not familer with that standard, so I will not that I am assuming that CIRCUITS referr to exernal equipment. Look the actual definintions up at the front.

The prefix at the start of a standard refers to the local standards authority. At the start of the document there should be a statement to say what the differences are (if any) from the IEC version. These differences can be due to local regulations (for example wireing colour), the fact the standard has been translated into a local language (which can introduce problems or inacuracies) etc. For example in the UK the prefix would be BS EN.

[Gyro]

Class II
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).

The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having at least two layers of insulating material between live parts and the user, or by using reinforced insulation.

In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol: ⧈ (a square inside another square).

Insulated AC/DC power supplies (such as cell-phone chargers) are typically designated as Class II, meaning that the DC output wires are isolated from the AC input. The designation "Class II" should not be confused with the designation "Class 2", as the latter is unrelated to insulation (it originates from standard UL 1310, setting limits on maximum output voltage/current/power).

i think most (mobile chargers) are just plastic right? so no earth chassis stuff..
plastic enclosures is one insulating material what is the second  insulating materials of the cell-phone chargers? is it the air between the wires/PCB and the plastic?

so what distinguishes (Class I) and (Class II) is that the chassis is insulation material (so not required to PE chassis for Class II) ?

Class II I'm not sure what counts as layers, possibly it's air inside the enclosure plus the enclosure thickness.  A failure of the enclosure isn't obvious, would that be cracking or fragmenting or completely falling off?  I'm not sure offhand how that's counted (and I'm sure it varies by standard).

Class II doesn't have to have a plastic enclosure. There are many consumer metal cased Class II products too - DVD players, Set-Top-Boxes etc. (I used to design them). These rely on double insulation and reinforced insulation of Primary - Secondary sides and Primary - case.

In these cases it's things like captive mains wiring so that it can't reach the chassis or secondary side, double insulation of mains wires - once they leave the outer cable sheath they are sleeved (usually heatshrink) and captive connector to PSU PCB. Compulsory use of Y1 caps. Appropriate design of SMPS to meet double / reinforced insulation requirements(triple insulated wire, large winding margins [Edit: in transformer] etc.)

Primary side PSU PCB to use creepage and clearances of minimum 8mm to case and fixings (reinforced insulation) or interposed insulation sheet + 4mm air gap (double insulation). Secure PSU mountings to guarantee 8mm clearance isn't compromised under mechanical shock. Secondary side wiring looms restrained so that they cannot  contact primary side (even if they come unplugged etc. Restricted case opening / ventilation slot sizes etc.

It sounds scary but it can be, and is routinely done. It's not something to self-certify without independent LVD test by a test house though.

[hsn93]

Why do you think SELV = PELV?

i just dont know the difference even after i read. so i went to check the standard definition and couldnt get the idea 

SELV includes protection to other circuits so the designer has to account for earth faults on anything that can be plugged in to it. PELV does not require this.

isn't that job of electrical panel (Residual Current circuit breaker). aren't these mobile chargers just a constant low voltage supply what they account for? i thought its just ELV so they dont care if someone touched the exposed conductor
+
its stated in many context that there is no need for this as its 'safe low voltage' so there shouldn't be leakage to earth through human body right. so why the designer would account for earth fault.


so what will make a 5vdc SMPS supply a (SELV device) or (PELV device) ? did i get that question right  ?


[edit: aaah, wait 5vdc could also be 100v away from earth if the 0v rail is not coupled to earth .. is this what its all about ?]