The issue arises because our instruments are for the industrial environment. They interface to low level sensors such as a thermocouple, RTD, magnetic pickup, flowmeter, etc. These are very low level signals in the 0 to 10VDC level or less typical. The output is also a low level signal in the 0 to 10VDC or less. For high performance and accuracy, we electrically isolate the input from the output and prevent ground loops from happening. In our installations, without isolation between the sensor and the control room equipment, the ground potentials are always different and if you don't isolate the signals become corrupted. The industrial environment always is an electrically noisy environment and isolating circuits is a way to help control the noise. It is not always necessary, and there are products that don't isolate input to output. There are hundreds of applications where the device won't work without isolation. But does the fact that isolation is included affect whether the Low Voltage Directive applies? I can make very strong arguments both ways that non-isolated devices are more hazardous than isolated devices due to the industrial environment they are installed in and vice versa.
Example 1: Isolated device input to output My sensor input is isolated from the output and grounded at the field sensor. My output signal of 0 to 10VDC is grounded at another point in the control room. Both signals are below 10VDC. However, the ground potential between input to output could be 250VAC. Does the Low Voltage Directive apply? Example 2: Non-isolated device from input to output My field sensor is grounded and connected to my input port. My output signal is grounded at the control room. Both signals are below 10VDC and input common is electrically connected to my output common. The ground potential at the sensor is 0 volts. THe ground potential at the control room is 250VAC with respect to the sensor ground. Hence a ground loop is set up and a 250VAC potential exists between the two points and current is flowing. Does the Low Voltage directive apply now? Or is should the end-user known better that they created this ground loop and hence now could be considered a hazard. Example 1 and 2 occur on a daily basis in the industry. Everyone has different opinions at this point. But the fact that the Low Voltage Directive applies or not when isolation is present has a major impact on the manufacturers product. Best Regards, Gary Labadie Marketing Acromag, Inc. 30765 Wixom Rd. Ph: (248)624-1541 ext. 205 P.O. Box 437 Fax:(248)624-9234 Wixom, MI 48393 Email: [email protected] -----Original Message----- From: James, Chris [SMTP:[email protected]] Sent: Thursday, June 11, 1998 3:43 AM To: 'Gary Labadie'; emc-pstc Subject: RE: Isolation versus IEC1010 The directive says "designed or adapted for use with......" the voltages ranges you state. If you product is not intended for use with such voltages then I do not see you need to seek approval (LVD) irrespective of what you rate the isolation. Having said that if you state a rated isolation then you need to be sure you meet it. How is your unit powered? Sounds like an external wall brick adaptor - this will of course need to comply. Chris > -----Original Message----- > From: Gary Labadie [SMTP:[email protected]] > Sent: Wednesday, June 10, 1998 8:08 PM > To: [email protected] > Subject: Isolation versus IEC1010 > > I would like to get the consensus on the following concept. > > The low Voltage directive applies to products that use 50VAC or 75VDC > and > higher signal levels basically up to 1000VAC. Now, If a product's I/O > > and power signals do not exceed 24Vdc anywhere, does the low voltage > > directive apply. Here's the catch, the unit contains electrical > isolation between input, output and power signals. The electrical > isolation rating is specified as 250VAC continous. > > Some argue, that since the unit is isolated and has an isolation > rating > the Low voltage directive applies because the isolation specification > > exceeds 50VAC. You can also argue that within the scope of the Low > Voltage directive that the I/O and power signals are below the > directives > requirement and hence do not apply if you don't consider whether the > > product has electrical isolation or not. > > Does the fact that electrical isolation is included into a product, > affect whether the Low Voltage Directive may or may not apply? > > Best Regards, > > Gary Labadie > Marketing > > Acromag, Inc. > 30765 Wixom Rd. Ph: (248)624-1541 ext. 205 > P.O. Box 437 Fax:(248)624-9234 > Wixom, MI 48393 Email: [email protected]
