The definition of SELV in IEC 60950 contains no restrictions on energy;
rather, voltage is the only issue. A note in clause 1.2.8.6 indicates that
IEC364 has a different definition of SELV. Is SELV defined to include energy
restrictions in some other standards?
Richard Woods
----------
From: Rob Legg [SMTP:r...@potentia.ca]
Sent: Friday, May 18, 2001 8:49 AM
To: Richard Meyette; 'Tania Grant'; emc-p...@majordomo.ieee.org
Subject: RE: Disconnect Devices for Telecom Equipment
Group,
Continual reference to centralized DC power systems as SELV is in
error. Although these systems may be ELV (isolated from hazardous voltages),
they are not SELV unless energy limited at the terminals so labeled. This is
typically by means of fuse, breaker or other suitable disconnect device.
Rob Legg
Potentia Telecom Power
200Katimavik Rd
Kanata K2L 4A2
Canada
r...@potentia.ca <mailto:r...@potentia.ca>
-----Original Message-----
From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Richard Meyette
Sent: Thursday, May 17, 2001 1:22 PM
To: 'Tania Grant'; 'emc-p...@majordomo.ieee.org'
Subject: RE: Disconnect Devices for Telecom Equipment
Tania,
The -48V centralized DC power systems used in the central
offices of telecommunications companies are limited to 60 VDC or less under
normal operating conditions, including during charging of the batteries, and
are therefore considered to be SELV by definition. See 3.6 of UL 60950.
Annex NAB clearly specifies that the connections to the
centralized DC power system must meet the requirements for primary circuits
as specified in section 3.2, however there are no requirements for
disconnect devices or overcurrent protection specified in this annex.
This equipment has operator replaceable fuses for over
current protection on the power inputs and has numerous overcurrent devices
in the PC assemblies. However, it does not have a disconnect device since
it is intended for installation into telecom racks with other equipment that
is similarly connected.
A disconnect device is required for equipment connected to
the AC mains, as specified in 2.7 of UL 60950, however there no requirements
for disconnect devices for equipment connected to a secondary SELV source.
The centralized DC power systems installed in these
restricted access locations are provided with disconnect devices for
overcurrect and fire protection. I'm not sure how you would specify
the short circuit protection requirements for these systems,
since they are not branch circuits covered under Article 240 of the National
Electrical Code.
Rick Meyette
-----Original Message-----
From: Tania Grant [mailto:taniagr...@msn.com]
Sent: Wednesday, May 16, 2001 7:18 PM
To: Richard Meyette; 'emc-p...@majordomo.ieee.org'
Subject: Re: Disconnect Devices for Telecom
Equipment
Rick,
A -48 volt source is considered a secondary circuit,
but that does not necessarily make it a SELV circuit. I also don't believe
that even if your equipment installation should be proven to be powered by a
SELV circuit that the disconnect requirements of Section 2.6 do not apply.
Here is why.
Note that UL 60950 plainly states that connections
to - and overcurrent protection for -d.c. powered equipment needs to meet
the same sections as for a.c. main powered equipment. It stands to reason,
therefore, that disconnection from whatever mains should also meet Section
3.4.3 (UL 60950) or 2.6 (I presume UL1950. 3rd edition).
Annex NAB.2 states that "... connections to the
centralized d.c. power system are subject to the requirements for PRIMARY
CIRCUITS and shall be in accordance with 3.2 (AC MAINS SUPPLIES)".
Sections 2.7.1 and 3.2.1 in Annex NAE which address requirements for d.c.
powered equipment refer you back to a.c. mains powered equipment. Also,
reading the standard as a whole, Permanently connected equipment, whether
a.c. or d.c., needs to meet the stated requirements. (All my references
are to the UL 60950 edition.)
There are many reasons why it can be considered
unsafe not to have immediate access to a disconnect device for any
equipment;-- a shock hazard being just one of them. Thus, equipment
powered from a SELV circuit is still subject to fire and/or an energy
hazard. For fixed equipment, the presumption is that the disconnect device
is in the building installation accessible to trained service personnel and,
therefore, the relaxed requirement to provide this information in the
installation manual.
However, I have a larger concern. If you don't
have a MAIN disconnect device in your d.c. powered equipment, something
tells me that you probably don't have a circuit breaker for overcurrent
protection. Annex NAE refers you back to Section 2.7.1 which states that
"... If...PERMANENTLY CONNECTED EQUIPMENT relies on protective devices in
the building installation for protection, the equipment installation
instructions shall so state and shall also specify the requirements for
short-circuit protection or overcurrent protection, or, where necessary, for
both."
I strongly recommend a good read of the complete
Annex NAE. Note that UL 60950 has renumbered or reassigned the Section
references; the content, however, has not changed from UL 1950 that I have
noticed.
Tania Grant
taniagr...@msn.com <mailto:taniagr...@msn.com>
----- Original Message -----
From: Richard Meyette
Sent: Wednesday, May 16, 2001 4:35 PM
To: 'emc-p...@majordomo.ieee.org'
Subject: Disconnect Devices for Telecom
Equipment
Here's a question for you telecom experts:
Paragraph 2.6.3 of UL 1950 (Third Edition)
requires permanently connected equipment,
that is not provided with a disconnect
device, to include a statement in the installation
instructions that an appropriate disconnect
device shall be provided as part of the building
installation.
Telecom equipment intended for connection to
a -48V centralized DC power system located
in a central office is considered to be
permanently connected equipment.
However, the centralized DC power source is
considered to be a SELV secondary circuit.
I don't think that this requirement would be
applicable in this case, since section 2.6 deals
with the requirements for primary power
isolation. However, I am being told otherwise.
It is no big deal to provide the statement.
I would just like to get some clarification
on how this requirement is applied to
telecom equipment.
Rick Meyette
Terawave Communications
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