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- Do the sheaths of all Pyrotenax Cables comply with the thermal constraint requirements of the IEE Wiring Regulations: BS 7671?
- Is it permitted to bury Pyrotenax Cables directly into concrete or plaster?
- Can I use bare copper Pyrotenax Cable on a galvanised cable tray?
- How do Pyrotenax Cables react if they are subjected to radiation?
- On some Pyrotenax Cables that I have installed, the sheath of the cable seems to run quite warm. At what temperature does Pyrotenax normally operate?
- What value of insulation resistance should I expect on a Pyrotenax Cable after installation?
- Are there any problems with running or installing the large single core cables?
- How long will Pyrotenax cable last in service?
- What is meant by 'not exposed to touch ratings' for Pyrotenax cables?
- Are Pyrotenax cables fireproof?
- Can Pyrotenax cables be used in potentially explosive atmospheres?
- Is it correct to say that Surge Suppressors are always required when Pyrotenax MI Cable is installed?
- Can Pyrotenax cable be buried directly in the ground?
- What is the minimum bending radius for Pyrotenax cables?
Q. Do the sheaths of all Pyrotenax Cables comply with the thermal constraint requirements of the IEE Wiring Regulations: BS 7671?
A. Yes, the copper sheath of all multi-core Pyrotenax cables is in every case of sufficient cross sectional area to meet the requirements of Table 54G of the IEE Wiring Regulations without verification by calculation.
Whilst the above is not always the case with individual single core cables the fact that 2 or more cables are always used, effectively increases the sheath CSA thus ensuring compliance with table 54G.
Q. Is it permitted to bury Pyrotenax Cables directly into concrete or plaster?
A. Firstly, when cables are buried in concrete we would recommend the use of plastic covered cables as the concrete mix may be corrosive to copper.
If the cables are to be buried in plaster we would have no reservations about using bare copper cables as normal wall plasters are not corrosive to copper. The cables would not normally require any further mechanical protection when buried in plaster or concrete.
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Q. Can I use bare copper Pyrotenax Cable on a galvanised cable tray?
A. Electrolytic action may take place when the coppersheath is in contact with the galvanised (zinc) plating of the cable tray.
The reason for this is that the electropotential series indicates that zinc is anodic to copper and therefore preferential corrosion of the zinc plating may occur. This action will not effect the Pyrotenax cable sheath, but may cause superficial corrosion of the cable tray.
The presence of moisture is essential to produce electrolytic action, therefore, in dry conditions this action will not occur.
If moisture is present, then electrolytic action will take place, but the extent of any corrosion is dependent upon the relative areas of the two metals and the conductivity of the electrolyte (moisture in this instance). In the case of Pyrotenax cable on a cable tray, the area of the cable sheath is small compared with the area of the tray, therefore the degree of corrosion will be low.
In conclusion, electrolytic action will not occur in dry conditions, but damp conditions may result in the tray deteriorating more rapidly than expected. The use of bare copper sheathed Pyrotenax cable on galvanised tray is therefore not normally a problem.
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Q. How do Pyrotenax Cables react if they are subjected to radiation?
A. The cable is manufactured from inorganic materials (copper and magnesium oxide) which make the cable highly resistant to radiation. The seals on the end do have organic compounds but we have successfully tested seals to a radiation dose of 950 megarads without losing their sealing properties. Such a seal consists of a 20 or 25 mm standard seal (ref. RPS) with the standard PVC sleeving replaced by silicone elastomer glass braided sleeving (ref. RZPS).
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Q. On some Pyrotenax Cables that I have installed, the sheath of the cable seems to run quite warm. At what temperature does Pyrotenax normally operate?
A. The exposed to touch current ratings of Pyrotenax cables are based on the sheath temperature of the cable rising by 40˚C. The ambient temperature is assumed to be 30˚C therefore the sheath can attain 70˚C when fully loaded. 70˚C is quite warm to the touch of course but this is quite normal and acceptable.
If required an estimation of the expected sheath temperature at any level of loading can be calculated using the following formulae, where
Ib = Load to be carried (amps)
Iz = Effective rating of cable (amps)
For Bare Cables Not Exposed to Touch
Approximate sheath temperature in ˚C
= (Ib/Iz)2 x 75 + Ambient temperature
For Cables Exposed to Touch or Plastic Covered
Approximate sheath temperature in ˚C
= (Ib/Iz)2 x 40 + Ambient temperature
e.g. A cable to be run singly, clipped direct and exposed to touch in an ambient of 30˚C
Load to be carried = 30 amps
Cable selected CCM2L4, tabulated rating 40 amps.
In stated conditions Iz = 40 amps.
Approximate sheath temperature in ˚C
= (30/40) 2 x 40 + 30˚C
= 52.5˚C
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Q. What value of insulation resistance should I expect on a Pyrotenax Cable after installation?
A. Firstly, the cable should not be tested until permanent seals have been fitted to both ends of the cable. An insulation test carried out 24 hours after the cable has been terminated, should achieve a value in excess of 100 MΩ.
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Q. Are there any problems with running or installing the large single core cables?
A. When single core cables are carrying currents in excess of 200A, considerable heating effect can be developed in associated switchgear due to Eddy currents generated in ferrous plates through which the cables pass, e.g. gland plates.
In dry conditions this heating effect can be reduced by cutting a slot between the gland holes in the box. Alternatively, or in damp conditions a brass or other non-ferrous gland plate can be fitted to the box.
Sheath circulating currents need not be considered if cables are installed in one of the standard configurations shown on page 70, as the effects of such current are considered during the current rating calculation.
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Q. How long will Pyrotenax cable last in service?
A. As previously stated, the cables are made from inorganic materials and therefore do not deteriorate with age. There are many installations throughout the world that are over 50 years old and the cable is in perfect condition.
In some cases, on very old installations the original neoprene conductor insulating sleeves may have deteriorated but it is a comparatively simple matter to renew these with the present day standard sleeving. Advice on this matter can be obtained from our Prescot address.
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Q. What is meant by 'not exposed to touch ratings' for Pyrotenax cables?
A. The current ratings usually referred to as for 'Bare cables exposed to touch or having an overall plastic covering' result in a 40˚C rise in sheath temperature above an assumed 30˚C ambient giving a sheath temperature of 70˚C.
70˚C is taken as the maximum continuous sheath temperature that can be permitted for cables which may be in contact with combustible building materials or human flesh. However, where cables are installed without a plastic covering and not in contact with combustible building materials, the Regulations permit their operation at higher temperatures provided that the loaded cables are protected from human contact.
Alternative sets of ratings for 'Bare cable not exposed to touch or in contact with combustible materials' are based upon a 75˚C rise from a 30˚C ambient (105˚C sheath temperature) and are given in table 4J2A of the IEE Regulations. In such installations the cable selection procedure is different from that described in this publication because it is not necessary to derate for bunching.
For operation at temperatures above 105˚C, which is the limit of operation for the standard RPS/RPSL seals, higher temperature seals are available.
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Q. Are Pyrotenax cables fireproof?
A. Pyrotenax cables will continue to operate in a fire at temperatures approaching the melting point of copper (1083˚C).
Whilst certain organic insulated cables claim to be 'fire resistant' or 'high temperature', Pyrotenax cable is truly fireproof.
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Q. Can Pyrotenax cables be used in potentially explosive atmospheres?
A. There is no problem using Pyrotenax cables with any apparatus certified or approved for use in Group II atmospheres.
The fact that the cable is fireproof, non-aging, resistant to mechanical abuse, etc. makes it ideally suited to these environments.
The fittings used on the ends of the cable must be carefully selected dependent on the 'type of protection' being employed.
For 'flameproof' installations (Type of Protection 'd').
The standard Pyrotenax male threaded gland Ref. RGM is certified to Cenelec Standard EN50 014 and EN50 018, BS 5501 Parts 1 and 5, and is suitable for use in Zones 1 and 2 with apparatus groups IIA, IIB and IIC. For selection of suitable seals.
For increased safety (Type of Protection 'e') installations, a special seal must be used on the end of the cable. The RGM and RGU glands are suitable for increased safety applications.
For 'intrinsically safe' installations (Type of Protection 'i') the RGM or RGU gland may be used with either the RPS or RPSL seal.
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Q. Is it correct to say that Surge Suppressors are always required when Pyrotenax MI Cable is installed?
A. No. This is a misconception.
There is a considerable misunderstanding with respect to the voltage performance of Pyro Mi Cables but here are the facts: The cables are manufactured and tested in accordance with BS 6207. The 500V cables are tested at 2kV a.c. (or 2.83 kV d.c.) and the 750V at 2.5kV a.c. (or 3.54 kV d.c.) so there is a significant safety margin. For more detailed information on the subject see overleaf.
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Q. Can Pyrotenax cable be buried directly in the ground?
A. Pyrotenax cables are suitable for burying in the ground and are subject to the same procedures as other cables regarding the depth of trench, mechanical protection and identification of routes. We would always recommend plastic covered cable as some soils can contain elements corrosive to copper.
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Q. What is the minimum bending radius for Pyrotenax cables?
A. The usual minimum bending radius specified is equal to six times the diameter of the cable sheath. This will allow the cable to be re-bent if necessary.
If the cable is being installed in such a situation that rebending will never be required, then the bend radius may be reduced to a minimum of three times the diameter.
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