Hazardous areas - control voltage transformer earthing -

[Shweblet]

I've been in an ongoing debate regarding earthing of a control transformer and i'd like some other opinions.

690 primary, 120v secondary supplying control circuits (contactors/DC convertors etc), the neutral is tied to earth - My issue is the location that it is tied to Earth.

As far as I know the difference between a TN-S and a TN-C-S system is whether or not the neutral is connected to 'ground' at the source or further down the line.

60079-14 - 6.3.2 TN type of system earthing If a type of system earthing TN is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of transition from TN-C to TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area.

Please see attached photo; The proposed design shows the point where the neutral is tied to earth (circled in blue) - this makes the highlighted section the 'C' in a TN-C-S system.

To create a TN-S system it would need to be where it is marked Red (taken directly off the transformer)

Also the highlighted cable is a line bushing from an Exd enclosure (which houses the transformer) to a close coupled Exe enclosure so this bushing cable would be combining the neutral and protective conductor.

Any thoughts agreeing/disagreeing? I can't seem to budge some peoples stance on it and to me it's black and white.

Cheers.

 

[buzzlightyear]

Erection of earthing arrangements (TNC, TN-S, TNC-S, TT) | EEP

For protective conductors of the same material as the phase conductor the cross-sectional area shall be the same size as phase conductor up to 16 mm2

electrical-engineering-portal.com

 

[Shweblet]

Thanks buzzlightyear but that's not what i'm getting at, I know you CAN do it especially in generally 'safe' area, but my point is that by not doing it at the source, it then becomes a TN-C-S, which is not allowed in a hazardous area... I'm having more 'experienced' guys tell me its ok and its always been like it, but as far as i'm concerned its crystal clear and I massively disagree. Just wanted to see what other people thought of it.

 

[Darkwood]

Are you not mixing Distribution network terminology ie your incoming DNO supply with what is effectively an isolated control supply here, I wouldn't be applying the terms TNCS etc to the control circuit post isolating TX, I have no experience of hazardous areas and circuits but have of machinery control and applying those labels is not really done, is the regulation referring to the incoming building supply?
If you are correct on using these terms then I would agree with your concerns, this would allow for the functioning of any specified leakage monitoring devices at circuit source, I do not know the regulations for circuit monitoring in hazardous areas but it may give incite to what is needed.

 

[Shweblet]

Hi Darkwood,

To be honest, this is why i'm asking. You may be absolutely right. That's what forums are for.

The protective device for the control circuit in this case is a 2A type C breaker (ignore the drawing that is just for reference). My main concern is; if for example the connection is made in a terminal box instead of directly from the transformer, there is more chance for an incorrect connection/may become loose/they may use a terminal link to make the connection which could be removed (I know its not allowed, but I've seen it), you are then leaving the installation at risk, if there was ever an earth fault from any of the control supply it would not have a path back through the transformer to the breaker and therefore spark which could ignite an explosive atmosphere.

I also think an RCD would be a good idea but I wont get started on that.

Also, the earth and neutral are still combined in a single conductor, or would you not agree with that terminology?

The regulation (6) is titled 'Protection from dangerous (incendive) sparking, 60079-14 is Explosive atmospheres: Electrical installations design, selection and erection.

Full extract below:
6.3 Danger from exposed and extraneous conductive parts
6.3.1 General
The limitation of earth-fault currents (magnitude and/or duration) in frameworks or enclosures and the prevention of elevated potentials on equipotential bonding conductors are essential for safety.
Although it is impracticable to cover all possible systems, the following applies to electrical systems, other than intrinsically safe or energy-limited circuits with voltages up to 1 000 V a.c. r.m.s./1 500 V d.c.
6.3.2 TN type of system earthing
If a type of system earthing TN is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of transition from TN-C to TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area.

Cheers,
Dan

 

[Darkwood]

I would have said you strap down at source ie the secondary of the TX, when I do so myself in control panels it is merely for ease to check voltages along the system and fault find and there is no reason for me (using 24v) to run a separate earth wire around with the control as it is common to the incoming earth to which in most cases bulk of the machine is earthed through various other connections from the power side, there is also no reason for me to strap it down either as this then acts as a isolation TX where you can only receive a shock by touching both legs at once but been selv then this is of no concern.
You however are using 110v where the strapping down is done part way into the circuit, I see no benefit to this and actually added risks even for a safe zone, you effectively have created a TNC for part of the circuit run, now this is definitely not permitted on the distribution side of your installation but we are not falling under those regulations here, the design will follow BS60204 if it is machinery control and any hazardous area regulations would take priority where there is confliction.

What is this control for and why not intrinsically safe, what category is the hazard zone?

 

[Darkwood]

The fact your info posted about TN systems expresses to equip' bond where TNC converts to TNS would suggest this is discussing the distribution side of the install to me, this would not cover the secondary side of a control panel hence my original confusion why you are applying language normally associated on the Network distribution and termination into the premises.

 

[netblindpaul]

Ignoring the hazardous area bit for a moment.
You have a control transformer, which if it complies with the relevant IEC standards isolates the primary and secondary.
Thus, you no longer have the concept of Live and Neutral with relation to the public supply.
Yes, one of the legs of the control Tx must be tied to earth, and direct from the Tx is the best place, but, if it is remote, you still do not have a TN-C, or TN-C-S supply.
You have a supply fed from an isolated control transformer.
The terms TN, TN-C & TN-C-S have no relevance to this supply when compared to the public distribution network.
Thus, your reference to these is irrelevant.

 

[timbobelfast]

I've been in an ongoing debate regarding earthing of a control transformer and i'd like some other opinions.

690 primary, 120v secondary supplying control circuits (contactors/DC convertors etc), the neutral is tied to earth - My issue is the location that it is tied to Earth.

As far as I know the difference between a TN-S and a TN-C-S system is whether or not the neutral is connected to 'ground' at the source or further down the line.

60079-14 - 6.3.2 TN type of system earthing If a type of system earthing TN is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of transition from TN-C to TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area.

Please see attached photo; The proposed design shows the point where the neutral is tied to earth (circled in blue) - this makes the highlighted section the 'C' in a TN-C-S system.

To create a TN-S system it would need to be where it is marked Red (taken directly off the transformer)

Also the highlighted cable is a line bushing from an Exd enclosure (which houses the transformer) to a close coupled Exe enclosure so this bushing cable would be combining the neutral and protective conductor.

Any thoughts agreeing/disagreeing? I can't seem to budge some peoples stance on it and to me it's black and white.

Cheers.

Are you sure the secondary side of this tx is not centre taped to earth ie 55 - 0 - 55v?

 

[Deleted member 26818]

If it’s centre tapped to earth, then you would not be wanting to earth a conductor elsewhere. You would just be creating an earth fault.

For the OP, yes tying the Neutral to earth remotely from the transformer, would make the system TN-C up to that point.

 

[static zap]

Are you sure the secondary side of this tx is not centre taped to earth ie 55 - 0 - 55v?

How would we test for this ?
a) 2x identical suitable value Lamps , resistors in series . See if mid point is 0V ?
b) (or just load 55v to earth for possible problems)

 

[Pete999]

How would we test for this ?
a) 2x identical suitable value Lamps , resistors in series . See if mid point is 0V ?
b) (or just load 55v to earth for possible problems)

Measure Voltage across output terminals and from each output terminal to CP of TX

 

[Deleted member 26818]

If it’s center tapped to earth, then you would measure 55V from any one conductor to earth.
If the Neutral is tied to earth, then you would measure 110V Line to earth, and 0V Neutral to earth.
If you cannot measure anything to earth from either conductor, then nothing has been tapped or tied to earth.

 

[davesparks]

How would we test for this ?
a) 2x identical suitable value Lamps , resistors in series . See if mid point is 0V ?
b) (or just load 55v to earth for possible problems)

Read the label on the transformer

 

[netblindpaul]

CTE is not allowed in the machinery directive electrical product standard, so if it’s CTE then it is non compliant and probably illegal as it will not meet the EHSR’s.