Earthing a PAR Can

[mikienorth]

Hello Everybody.

 

Just a basic question.

 

If a Par Can is fitted with a Parsafe (Parshell type to be pedantic) and the rear half of the lantern is bonded to earth, should the front half be bonded to earth as well?

 

We are in the process fo testing some of our older stock, and are busy arguing amongst ourselves, so if anyone can help me win the argument (or otherwise) it'd be ace!

 

Mike

[david.elsbury]

On a slight tangent, often it is the earth bond that provides the link between the front and the back of the can, so that when you remove the back it doesn't come off. (unless you have decent, hinged cans of course )

 

So would this be an acceptable argument, to leave the bonds in place?

 

In practice, if the lantern is being tested as a double-insulated device, I see no reason for a secondary earth bond to be fitted to bond the front and the rear halves. Of course, YMMV

[mikienorth]

As the back of the lantern is earthed, I am unable to test it as a Class 2. Should I have the front of the can earthed? It is possible to do so, but I would like an opinion before I go about it.

[dirkenstein]

As the back of the lantern is earthed, I am unable to test it as a Class 2. Should I have the front of the can earthed?

 

OK- You're using class I parsafes, which actually have a three-wire cable and a specific earth bonding wire to the rear body of the lantern.

 

How old are these cans, and is the back body shell hinged or held on by a restraint wire?

 

There has been argument in the past about the safety of retrofitted class II parsafes as these seemed to discount the possibility of the parcan body itself becoming live. This would be in a fault scenario where the PAR lamp shatters and one of the filament connections contacts the body of the PAR without actually shorting out. This is admittedly an unlikely, but not particularly implausible scenario.

 

Unless the metal screen (which is meant to protect from falling glass when a PAR lamp shatters) is part of the rear lamp assembly (I've never seen a can with this design), I would say the front of the can needs to be earthed.

Generally class I devices must have all exposed conductive parts bonded to earth.

The question is, is the front of the parcan an exposed conductive part?

The usual (16th ed.) definition is:

"exposed conductive part is a conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions," (P 11, 16th edition, Wiring Regulations).

 

I don't know if this has changed in the 17th ed.

 

So unless specific precautions were taken to prevent the front of the can from becoming live under fault conditions, such as double insulation or other physical separation between the rear body and front body of the can, I would say the front of the can is an exposed conductive part.

 

In addition, I would expect some protection aginst exposed parts of a shattered lamp contacting the front body of the can to be necessary for it not to be treated as an exposed conductive part.

 

Remember that unlike in ES or BC cap d0mest1c lamps, where the filament supports are held on a glass support rising out of the cap base (and any shade or lamp body is attached to the lamp base), the PAR lamp (and its filament support) are held in physical position inside the parcan by the glass body of the lamp reflector, which is one of the parts most likely to break when a lamp shatters. This means a shattered PAR lamp is quite likely to shift position inside the parcan, making it that much more probable that any live filament supports exposed by the lamp shattering will actually contact the body of the parcan.

 

In addition, the fact that the front of the can would generally have some (unspecified, potentially poor) electrical contact with the yoke and hookclamp, which in practice would be mounted on an earth-bonded lighting bar, would suggest to me that it could become an extraneous conductive part if not earth bonded via its cable. In that case, it would actually in all probability be safer that neither the front nor the rear body of the parcan were earth bonded, provided the parsafe itself was a class II type.

 

The one exception I would allow to the front of can being earth bonded, and I say this with some reservations, is if you are looking at a can from a reputable manufacturer that originally fitted the parsafe as part of the design and did not consider it necessary to include an earth bond between the front and the rear shell. In that case it's quite possible the manufacturer actually asessed the risk and mitigated it via some other non-obvious design feature. In either case I'd consult with the original manufacturer and request their response in writing. It's quite possible the earth bond has gone missing, for instance, and was fitted at original manufature.

 

If you or somebody else retrofitted the parsafe, or the parcan is one of many generic models (of varying quality standards) manufactured in the far east over the last few years, I wouldn't make that assumption unless you are absolutely confident the retrofit has not impacted the design safety.

 

As you're asking this question in a public forum, rather than, say consulting your in-house safety engineering people or the relevant BSI/EN/IEE specifications, I would suggest that you cannot make such a confident statement in a qualified manner.

 

You can of course risk assess the possibility of any live component of a shattered lamp or damaged internal wiring touching the front of the can as minimal or not worth being concerned about, but I would personally be extremely reluctant to do so without either being able to argue that the design is safe from basic engeineering principles or having experimental and statistical evidence indicating such a fault to be very unlikely, especially since PAR lamps are known to occasionally (if infrequently) fail by shattering in a quite dramatic fashion.

[mikienorth]

OK- You're using class I parsafes, which actually have a three-wire cable and a specific earth bonding wire to the rear body of the lantern.

 

How old are these cans, and is the back body shell hinged or held on by a restraint wire?

 

Neither, the cans have no way of holding the two bits together...

 

<snip>

 

Thankyou...

[Ynot]

Hi Mike.

 

I'd look at it this way....

Presumably the cable is still going through the rear body of the lantern, yes?

So there remains the risk of cable being damaged at or near that point and live coming into contact with the metal case.....

Ergo, it's necessary to have the rear bonded to earth.

And if that in itself is feasible, them the front half of the can could equally be raised to live potential whilst said cable is in contact, therefore the front needs bonding to earth.

 

Personally the fact that the wire bond bewtixt the front and back will also act as a restraint is enough for me to weigh down on the side of caution. Stick a bond on - but make sure it's long enough!! :)

[Roderick]

If you are using the Admiral ParSafe (or the Kupo copy) then there should not be an earth connection at all.

When correctly installed, the ParSafe is now a Class 2 instrument which should not have an earthbond.

The installation instructions are actually quite clear about that.

[Brian]

I'd look at it this way....

...

And if that in itself is feasible, them the front half of the can could equally be raised to live potential whilst said cable is in contact, therefore the front needs bonding to earth.

But the front half could not become live without the earthed rear part becoming live so your fuse/trip would operate anyways.

 

And what 16th/17th editions say is completely irrelevant as they do not apply here.

[techtech]

Hello Everybody.

 

Just a basic question.

 

If a Par Can is fitted with a Parsafe (Parshell type to be pedantic) and the rear half of the lantern is bonded to earth, should the front half be bonded to earth as well?

 

We are in the process fo testing some of our older stock, and are busy arguing amongst ourselves, so if anyone can help me win the argument (or otherwise) it'd be ace!

 

Mike

 

Yes both parts of a parcan should be bonded. It is safer and good practice. :)

 

TT

[grizzly]

Generally class I devices must have all exposed conductive parts bonded to earth.

 

Generally maybe, but not exclusively.

Class I appliances may have unearthed metal as part of their construction.

Have a look at the 3rd edition IEE CoP, sections 11.1.3 & 11.1.4 (pages 55 & 56), also the ABTT CoP, section 3.3.1 (page 13).

 

The usual (16th ed.) definition is:

"exposed conductive part is a conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions," (P 11, 16th edition, Wiring Regulations).

 

I don't know if this has changed in the 17th ed.

 

As Brian said, these do not apply to this issue.

[Brian]

Being a bit pedantic but it's worth quoting the proper definition of a Class I device...

Class 1 Equipment

equipment where protection against electric shock is achieved by

-using basic insulation and

-providing a means of connection to the protective earthing conductor in the building wiring those conductive parts that are otherwise capable of assuming hazardous voltages if the basic insulation fails

[lightsource]

I'd look at it this way....

Presumably the cable is still going through the rear body of the lantern, yes?

So there remains the risk of cable being damaged at or near that point and live coming into contact with the metal case.....

Ergo, it's necessary to have the rear bonded to earth.

And if that in itself is feasible, them the front half of the can could equally be raised to live potential whilst said cable is in contact, therefore the front needs bonding to earth.

 

Personally the fact that the wire bond bewtixt the front and back will also act as a restraint is enough for me to weigh down on the side of caution. Stick a bond on - but make sure it's long enough!! :)

 

 

If you are using the Admiral ParSafe (or the Kupo copy) then there should not be an earth connection at all.

When correctly installed, the ParSafe is now a Class 2 instrument which should not have an earthbond.

The installation instructions are actually quite clear about that.

 

This is something I've been concerned about for a while. We have a quantity of DTS ParSafe Par56, and the cable entry to the lantern is not protected. The cable is also 2 core, so there's no provision to add an earth.

 

in an old similar topic.....

 

http://www.blue-room.org.uk/index.php?showtopic=9947

 

It seems to be suggested that part of the plastic of the ParSafe protrudes out of the back of the Can... Not in the ones we have.