Use of FP32 Surge Arrester

[labingtone]

Dear all,


Currently Im making a Fieldbus quotation for geothermal exploration project here in the philippines. There is something that amaze me on there segment specification. Here is the detail:

1. both end of Homerun cable is equiped with FP32 Arrester.
2. There are 5 devices installed. The horizontal adjacent distance of each device is more than 100 meter. both end of the 5 Spur cable is euiped with FP32.

My question:
1. If there are 12 pcs of FP32 surge arrester in one segment does this amount of surge arrester weaken the Fieldbus Signal on the segment.
2. What is the maximum amount of surge arrester in 1 segment.

best regards,

LOVINGTON DELA CRUZ

[Mike ONeill]

The general rule for surge protection is that it only works if all the angles are covered. The damage occurs as a result of temporary but massive shifts in local ground potential, which only concerns you if your cable run goes across those areas. Surge protection schemes try to create local equipotential zones and to provide preferred paths for excess currents, which flow because those equipotential zones become temporarily different as a result of lightning strikes or local power faults.
The scheme you are describing has a surge protector at the local end, to divert surges coming down the trunk. At the other end, in the field, you are putting together another network of SPDs to maintain common potential voltages around the termination hub. In the field, the devices themselves need protecting.
The operational voltage of the FP32 is(should) be designed to be well above the maximum operational voltage on FF H1 and so any number of FP32's fitted should have no impact from a leakage current point of view. Check out to see if there is any series resistance within the FP32, but I expect it to be still well within the segment load parameters with only a 200m trunk.
At Hawke, we offer proper 3-element 10kA surge protection already built in to our FF power supply carrier, we incorporate a little surge protection in the field termination hub (TrunkGuard) and we recommend the TP48 to fit on the field devices themselves.
Mike O'Neill

[Chris Ground]

To answer your questions in reverse order, the maximum number of surge protectors which can be installed will depend very much on the surge protector, as Mike O'Neil has pointed out, and the length of fieldbus wiring required. For a very simple "rule of thumb" the FP32 is equivalent to 20m of field cabling and the TP32 35cm of field cabling. The FP32 has been designed to fully comply with the FF specifications and not to affect the operation.

Fitting 12 FP32's in your design will have minimal effect on the signal and will only act as if you have added more cable.

I hope this reply answers your questions, if you need any further clarification please do not hesitate to contact me directly or through this forum and I would be happy to look at the scheme layout and offer technical clarification.

Chris Ground
MTL Surge Technologies
cground@mtlsurge.com

[labingtone]

Chris Ground,

I have read your article on "SURGE PROTECTION FOR FIELDBUS SYSTEMS".







I just want to ask what is the minimum distance of cable between the H1 interface module and the surge arrester.




Many thanks,

Lovington Dela Cruz

[Chris Ground]

I realise this may appear to be a vague answer but the length of cable will depend upon the earthing/grounding arrangement adopted. If you adopt a policy of single point grounding (that is all of the system bonded with reference to one point and that point taken to ground once) then the length of the cable can be several meters or more. Other grounding arrangements will need more careful consideration as the wrong method may be very detrimental.


I would urge you to read MTL's TAN 1003 about grounding for surge protectors especially the practical steps/philosophy for grounding control systems.

Rgds

Chris Ground