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ticker
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Join Date: Jul 2012
Posts: 14

Ungrounded System Protection

10/09/2012 3:58 AM

Hi,

I am looking for information on French and American electrical standards and legislation relating to ungrounded dc floating supply systems, specifically relating to earth monitor protection systems. We have a production system manufactured by a French/American company which is a process electrodialysis installation, it has a 550v floating d.c. supply and load current of 400 amps. I am struggling to convince the suppliers of the requirement/ need to provide, what I believe to be basic personnel protection.

In the UK the wiring regulations covers this installation requirement under section 411 Protective Measure: Automatic Disconnection of supply, which more less states for an I.T. (floating) supply an earth monitor must be provided which must alarm when one pole develops an earth fault and disconnect the supply when the second pole becomes earthed.

I beleive that in the USA this requirement is covered in NEC 250.4 (B) but I am not sure whether or not this is regulatory requirement, I don't know anything about the French standards of protection.

If anybody has any information that might assist me in convincing the equipement manufacturer that this protection is essential please let me know.

regards
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electronick
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#1

Re: Ungrounded System Protection

10/09/2012 5:22 AM

strange that you are determining that the supply is intended not to be grounded. such practice is highly dangerous. In IT system´s the earthing is made by a high resistive path to earth, but still, ever a path to the ground

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PWSlack
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#2
In reply to #1

Re: Ungrounded System Protection

10/09/2012 6:14 AM

What makes it dangerous, please?

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TonyS
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#4
In reply to #2

Re: Ungrounded System Protection

10/09/2012 1:11 PM

The possibility of an undetected EF in one leg means the system is no longer "earth free."

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PWSlack
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#5
In reply to #4

Re: Ungrounded System Protection

10/10/2012 3:24 AM

Do go on. The danger is not yet apparent.

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electronick
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#6
In reply to #5

Re: Ungrounded System Protection

10/10/2012 5:08 AM

can you imagine that the mass of a apparatus became in contact with the main power and you put your hand´s on it? and the ground has a very different potential ?

in stead of PWSlack we would have a PWrouster!

is it more clear now?

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PWSlack
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#7
In reply to #6

Re: Ungrounded System Protection

10/10/2012 5:54 AM

Noted. So make the ingress protection rating >=IP2x so as to shield people from the hazard. Where is the "danger" now, please?

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ticker
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#8
In reply to #7

Re: Ungrounded System Protection

10/10/2012 9:59 AM

Guys thanks for your assistance, but its not much help I'm afraid.

The IEE wiring regs covers this very well for me, an earth monitor circuit which alarms on the first phase earth fault as the system has literally changed from a floating system to a TN-S, which requires to trip when the second phase is earthed.

The problem I have is that the manufacturers claim that in France and the USA (their manufacturing base) this protection is not required according to their rules, and therefore should not be fitted. Personally I can't see how this can be acceptable, but I don't know enough about the electrical regulatory requirements in these countries to understand where they are coming from.

I know the rules for the UK and will stick by them, however I thought if I could find out more about the specific rules that apply to France and America it would help strengthen my case to ensure we have a safe system.

Thanks again
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NotUrOrdinaryJoe
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#3

Re: Ungrounded System Protection

10/09/2012 10:13 AM

Just a thought. I would assume that you are creating the DC from some source of AC?

Because, if you don't have electrical isolation then you must have magnetic isolation. Think of it this way. If you ground one leg of your DC, then you are automatically half way to a short circuit. Perhaps what you need is sufficient isolation from any portion of the DC circuit. There is no magic protection from grounding a DC circuit. If you do decide to do it, you must protect branch circuits like you do for AC.

I think you will find your answer in 668.2(3) which says that electrolytic cell line working zone ... shall not be required to comply with the provisions of Article 250.

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JF-REY
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#9

Re: Ungrounded System Protection

10/10/2012 11:36 AM

Hello,

I am from France and I may provide some answers to your question. French Installation standard, known as NF C15-100, brings some requirements for IT system (you named it ungrounded system).

As the french standard is similar to IEC 60364, even if some small change are present, you may refer to IEC 60364-4-41 for your question.

Clause 411.6 deals with IT system and requires that IMD (Insulation Monitoring Device) be installed to detect that a fisrt fault occured. It also requires that the touch voltage do not exceed 120 V in dc (safety voltage in dc).

Additionnaly, you may have a look at the following wiki website which provides interesting information related to your question. I attached the link below.

http://www.electrical-installation.org/enwiki/Protection_against_indirect_contact

Regards,

Jean-François REY
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electronick (2); JF-REY (1); NotUrOrdinaryJoe (1); PWSlack (3); ticker (1); TonyS (1)

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