<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>125 - Disclosure - Concentration of revenues</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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&lt;table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"&gt;
&lt;tr&gt;
&lt;td width="4%" valign="top" align="left"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&lt;b&gt;17.&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
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&lt;p style="margin-top:6px;margin-bottom:0px; text-indent:4%"&gt;
&lt;font style="font-family:Times New Roman" size="2"&gt;The following is
a concentration of revenues greater than 10% by customer for the
periods shown:&lt;/font&gt;&lt;/p&gt;
&lt;p style="font-size:12px;margin-top:0px;margin-bottom:0px"&gt;
&amp;#xA0;&lt;/p&gt;
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&lt;tr&gt;
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&lt;td valign="bottom" width="6%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td valign="bottom" width="6%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td valign="bottom" width="6%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td valign="bottom" width="6%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
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&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="6" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;Three&amp;#xA0;Months&amp;#xA0;Ended&lt;br /&gt;
June&amp;#xA0;30,&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="6" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;Six&amp;#xA0;Months&amp;#xA0;Ended&lt;br /&gt;
June&amp;#xA0;30,&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="2" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;2013&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="2" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;2012&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="2" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;2013&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" colspan="2" align="center" style="border-bottom:1px solid #000000"&gt;&lt;font style="font-family:Times New Roman" size="1"&gt;&lt;b&gt;2012&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
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&lt;td valign="top"&gt;
&lt;p style="margin-left:1.00em; text-indent:-1.00em"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;General Electric
Company&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
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&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;13&lt;/font&gt;&lt;/td&gt;
&lt;td nowrap="nowrap" valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;%&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;13&lt;/font&gt;&lt;/td&gt;
&lt;td nowrap="nowrap" valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;%&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;13&lt;/font&gt;&lt;/td&gt;
&lt;td nowrap="nowrap" valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;%&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
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&lt;td valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;13&lt;/font&gt;&lt;/td&gt;
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&lt;td valign="top"&gt;
&lt;p style="margin-left:1.00em; text-indent:-1.00em"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;Royal Bank of
Canada&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
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&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;11&lt;/font&gt;&lt;/td&gt;
&lt;td nowrap="nowrap" valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;%&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family:Times New Roman" size="2"&gt;10&lt;/font&gt;&lt;/td&gt;
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