Electrical Separation for more than one item

Dean Williams · Jul 24, 2011

Hi gang
im currently testing a theatr in north wales.
ive come across a 1:1 isolating transformer feeding 32 bench sockets, it was installed in 1976. its in an audio/visual repair shop where they used to repair old tvs etc.

all the sockets have an earth
L-N measures 230v
N-E measures 110v
N-L measures 110v

is it the primary or the secondary winding thats centre tapped?, 2 people have told me different answers.

i've found it hard understanding the whole elec separation theory, any help, links to diagrams would be greatly appreciated.
thanks

IQ Electrical · Jul 24, 2011

To be a separated system, there should be no tap on the secondary or primary and no CPC on the separated side.

Dean Williams · Jul 24, 2011

thats what i originally thought, until i came across regulation 418.3.4
the exposed conductive parts of the separated cct shall be connected together by an insulated non earthed protective bonding conductor. such conductors shall not be connected to the main protective earth.

the sockets ive found have a 'protectove conductor'.
apparantly its to protect against the chance of 2 elec faults on the circuit, where in one fault its a live to earth short and in another its a neutral to earth short, meaning theres 230v between the two items

spark 68 · Jul 24, 2011

Re: electrical separation for more than 1 item

is it the primary or the secondary winding thats centre tapped?, 2 people have told me different answers

Hi it will be the secondary which will be tapped and earthed, very similar to a normal yellow site transformer, except this is tapped at 55 Volts as this is of course 110V

Dean Williams · Jul 24, 2011

Re: electrical separation for more than 1 item

thanks.

spark 68 · Jul 24, 2011

Re: electrical separation for more than 1 item

I have also seen some Isolation transformers where an earth connection is present on the primary side, just to the metal work,and not connected on the secondary at all. (when I worked in Electronics servicing/repair)

This may explain why you are getting different answers

IQ Electrical · Jul 24, 2011

Yes, exposed conductive parts must be joined electrically but this is not a cpc.

There's some conflicting information here, what BS are the outlets?

Dean Williams · Jul 24, 2011

not sure. they look like standard white plastic sockets and theyre in plastic dado trunking. the trunking is fed from the transformer in steel conduit. this cpc or whatever it is goes from the earth point on the steel conduit end box to all the sockets.
an audio repair engineer told me that only the primary side must be centre tapped to earth.

Dean Williams · Jul 24, 2011

Re: electrical separation for more than 1 item

that could be it. im told that old tv sets only had a neutral and live and that the neutral went to the chassis, which is why this system was installed

spark 68 · Jul 24, 2011

Hi IQ,
I gave two different answers to the OP's question in the T&I section, (in view of his readings), I do not know off hand what the BS numbers are.

I have seen both types of Isolation tranformers used in Electronic servicing.

Usually the type with no earth on the secondary (just to the metal work on the primary side).

The regs are a bit different for servicing live gear, we had to install dividers between bays (in case you hand your mate a screwdriver etc. while in contact with different potentials) these were 'earth free' locations and rubber mats throughout.

I have an Isolation transformer here, I will look later to see how that one is configured.

Dean Williams · Jul 24, 2011

thanks for your help. i'll have another look at the circuits tomorrow at work

spark 68 · Jul 24, 2011

Right, the one I have here is different to the bench ones.

The bench ones were wired in via an FCU, and boxed in a wooden enclosure (physically away from the work area).

This one is a 'self contained' portable unit, it has a three core cable and a normal BS1361 plug fitted, the earth on the socket is connected to the earth on the plug and the metal casing, and it is not tapped at all (either primary or secondary).

Of course neither side of the fitted socket is 'live' with respect to earth.
But this is a different type to the one the OP is asking about.

The bench fitted ones had the metal mass of the transformer connected to a CPC, and NO CPC on the output side as already said.

IIRC, we had a seperate one for the device under test (DUT) and one for the test equipment in each bay.

It was odd fitting sockets with NO CPC and no RCD, as of course these were supplied with the isolated supply, they were however labelled.
The CPC or RCD would not have worked anyway as there is no Neutral or Live wrt earth, out of the Isolation transformer, just 230V between the line conductors.

UKMeterman · Jul 24, 2011

Dean Williams said:
not sure. they look like standard white plastic sockets and theyre in plastic dado trunking. the trunking is fed from the transformer in steel conduit. this cpc or whatever it is goes from the earth point on the steel conduit end box to all the sockets.
an audio repair engineer told me that only the primary side must be centre tapped to earth.

If this an ISOLATION transformer then there must be no connection between any primary center tap and earth, this center tap would be at 120V (half supply) with respect to earth if the supply was of normal configuration and had a earthed neutral. If this was an AUTO transformer, single winding no isolation, then yes do connect the earthed point of the supply ( the neutral connection) to the center tap. In either case the green and yellow wire does not get connected directly to the primary windings.

I would suggest adding some MK 6216s RCDs 10mA trip even on pulseing dc faults, so will even trip if someone grabs the output of a faulty dimmer. Note that all circuit breakers on the output of the transformer need to be double pole.

spark 68 · Jul 24, 2011

Hi UkMeterman,
The OP also posted this in the T&I section,

I said it was possibly Earthed centre tapped on the secondary side (similar to the 55-0-55 110V site transformers) as I have seen one like this before, but they are usually arranged like either the portable one above, or the bench (hard wired) type above with no CPC on the secondary at all.

I agree with you that it cannot be centre tapped to Earth on the primary side.

Dean Williams · Jul 25, 2011

Re: electrical separation for more than 1 item

looked at it today. the cpc (or screen) goes from the metal casing of the transformer to all the sockets. there are no centre taps.
IR between the seperated earth and other circuit earths is 0Mohms (dead short).
spoke to NIC who say this complied when it was installed (15th ed).
note: when i plug martindale tester in it says no earth, which is right.

Electrical Separation for more than one item

Dean Williams

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IQ Electrical

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spark 68

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spark 68

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IQ Electrical

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Dean Williams

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spark 68

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